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How to Caption Photos for a Substack Newsletter
By Duncan Rawlinson · Updated
A Substack newsletter is part magazine, part letter from a friend, and the photos you include set the tone of which one it feels more like. Captions are the bridge between the visual and the voice of your writing, and getting them right makes a noticeable difference in whether readers stay engaged through a long issue.
Substack's editor handles images well but has its own quirks that catch newsletter writers off guard, especially around alt text, caption visibility in the email version, and how images render across mail clients. This guide walks through what actually works.
Whether you publish a personal essay newsletter with a few embedded photos or an image-heavy travelogue, the goal is the same: captions that add to the reader's experience without breaking the flow of your prose.
Substack Image Embed Quirks Worth Knowing
Substack's editor looks simple but does several things automatically that affect how your captions appear. Knowing the behavior in advance saves a lot of frustrated re-uploads.
Caption and Alt Are the Same Field
When you upload an image to Substack and click into the caption area, the text you type becomes both the visible caption and, in most cases, the image's alt attribute. This is convenient but it means a clever caption written for sighted readers may not function well as alt text for screen reader users.
If you want the two to differ, you can use the alt text field exposed by clicking the image options. Most writers do not bother, which is why so many Substack posts have alt text like "see end of issue for credits."
The Email Version Is Different from the Web Version
Substack renders newsletters twice: once for the web archive and once as an email. The email version is more constrained. Captions appear, but image positioning, gallery layouts, and full-bleed images often look different in Gmail and Outlook than they do in Safari.
Always send a test email to yourself before publishing an image-heavy issue. The preview in the editor is not reliable.
Image Galleries Hide Captions
The gallery embed combines multiple images into a slider, but the captions become hover text on the web and are usually stripped entirely from the email. If your captions are critical, post images individually with captions beneath each one. If the images can stand on their own, a gallery saves vertical space.
Captions vs Alt Text on Substack
Because Substack collapses these into one field by default, most writers end up with text that does neither job well. A good caption is short, evocative, and adds context. A good alt text describes the image so a screen reader user can understand what it shows.
Caption Style
Captions can assume the reader sees the image. They work best when they say something the image alone does not: who, when, why this moment, what happened next. A caption is a tiny essay.
"Maya, the morning after she finally said yes to the move." That kind of caption deepens the reader's connection to the image.
Alt Style
Alt text is for someone who cannot see the image. It needs to describe the visual content directly: subject, action, scene, notable details.
"A woman with curly hair sits at a kitchen table looking out a sunlit window, holding a mug of coffee." That description tells a non-sighted reader what is in the frame.
If you only have time for one, the visible caption usually wins because it serves both audiences imperfectly. If you write for a publication or have any subscribers using assistive technology, take the extra 30 seconds to fill the alt field separately.
Captions and Click Through
Substack tracks clicks on links inside your newsletter, and links inside captions perform well because readers tend to slow down to look at images. A caption with a single, well-placed link can outperform the same link buried in body text.
Link to Context, Not Promotion
Captions linking to a previous issue ("we last met Maya here"), a portfolio page, a reference, or an external resource feel earned. Captions linking to a paid subscription pitch or merch feel like an interruption. Save the promotional links for the dedicated CTA sections of your issue.
One Link Per Caption
Captions are short. Two or three links inside a one-sentence caption look cluttered and reduce the click-through rate on each one. Pick the single most useful destination.
Use Descriptive Link Text
"Read the original interview" performs better than "click here." Substack's analytics will show you which captions drove engagement, so you can develop a sense over time of what your audience responds to.
Image-Heavy Newsletter Best Practices
Some newsletters are essentially photo essays with text in between. Travel writers, food writers, and visual journalists all hit the same set of constraints: large emails get clipped, mobile readers scroll fast, and clients render galleries unpredictably.
Practical Rules That Hold Up
- Compress before upload. Substack does its own compression but starts from your original. A 12 MB file uploaded gets resized but takes longer to process and can push your email past Gmail's clipping threshold.
- Lead with your strongest image. The image at the top of the email becomes the social card and the inbox preview. Pick one that earns the open.
- Caption every image, even briefly. Uncaptioned images break the prose-image-prose rhythm and feel like dropped puzzle pieces.
- Vary the caption length. A long caption every now and then breaks the rhythm and gives the reader a place to settle. Three-word captions in a row feel terse.
If your email goes over Gmail's roughly 102 KB limit, the bottom gets clipped and replaced with a "view entire message" link. Image-heavy issues are the most common cause. Fewer, bigger images often beat many small ones for both performance and impact.
AI Caption Tone for Personal Newsletters
Personal newsletters are a tough place for AI-generated text. The whole point of a Substack subscription is that someone has chosen your specific voice. Generic, technically-correct captions break that contract.
Use AI as a Speed Layer, Not a Voice
The right way to use AI for newsletter captions is to generate a fast first draft and rewrite it in your voice. The AI tells you what is in the image and supplies the obvious context. You add the perspective, the joke, the memory, the half-formed thought that a real person would write.
Set a Style Preference Closer to Your Voice
If your newsletter has a recognizable tone, configure your tool to match it. PhotoScanr lets you set a style preference that biases all output toward a particular register, whether that is wry, lyrical, journalistic, or plainspoken. The closer the AI draft starts to your voice, the less editing you have to do.
Watch for Tells
Readers can sense AI captions even when they cannot articulate why. Common tells include hedge words like "appears" and "seems," lists of three things, and overuse of words like "vibrant" or "stunning." Edit those out aggressively. Real captions are usually more specific and a little stranger than what an AI produces by default.
How PhotoScanr Fits a Substack Workflow
Most newsletter writers do not need a heavyweight photo management tool. They need a way to caption a handful of images quickly without losing their own voice in the process.
Single-Issue Workflow
For a typical issue, drop the images for the post into PhotoScanr, generate captions and alt text, copy the text into your Substack draft, and edit each one to add your voice. The whole process takes a few minutes per issue once you have a style preference dialed in.
Style Preferences Save Time
Spend half an hour writing a detailed style preference once. Tell the tool your typical voice, your typical caption length, the words you avoid, and the perspective you write from. Every batch you run after that arrives much closer to publishable.
Batch Mode for Image-Heavy Issues
Travel issues and photo essays can run 30 to 80 images. Pro covers 100 images per day with batches of 25, which is enough for most issues. Studio covers 600 per day with batches of 100 and is overkill for newsletters but useful if you write multiple publications.
If you are scaling up to weekly or daily image-heavy issues, see our bulk captioning workflow guide.
Putting It All Together
Captions on Substack are not just labels. They are part of the prose. A good caption deepens the moment, adds context the image cannot carry, and gives a reader a place to pause inside a longer post.
Know the platform's quirks: alt and caption are usually the same field, the email version differs from the web, and galleries lose their captions in most clients. Plan your image strategy accordingly.
Use AI to handle the tedium of describing what is obviously in the image. Reserve your own attention for the part that is not obvious: the why, the when, the small detail that makes a reader hit reply.