Substack distributes your newsletter both as a web page and as an email — two very different rendering environments. An image that looks sharp on the web version may be too large for email delivery, and Gmail clips emails exceeding roughly 102 KB total. Getting your image sizes right ensures your Substack newsletter looks professional everywhere and reaches every subscriber's inbox intact.

Substack Image Sizes — Quick Reference

Post cover (featured image): 1000×563 px (16:9 ratio) — JPEG for photos, PNG for graphics, under 200 KB
Inline images: upload at 1600 px wide, Substack scales to 800 px — under 150 KB each for email delivery
Profile photo: 400×400 px (1:1 square, displayed as circle) — PNG or JPEG
Publication logo: 256×256 px minimum — PNG with transparent background preferred
Custom header image: 1500×500 px (3:1 ratio)
GIFs: supported — keep under 5 MB

Why Image Size Matters for Substack

Substack is unique among publishing platforms because every post is simultaneously a web page and an email. The web version renders at full quality using Substack's CDN, but the email version is subject to the constraints of every subscriber's email client.

Gmail clipping: Gmail truncates emails over approximately 102 KB total and replaces the rest with a "View entire message" link. Images hosted on Substack's CDN do not count against this limit directly, but excessively large inline image files increase overall email size and load time.
Email client variability: Outlook, Apple Mail, and Gmail each render images differently. Uploading at the correct dimensions prevents unexpected scaling or overflow.
Mobile subscribers: More than half of email opens happen on mobile. Smaller inline images load faster on cellular connections.

Post Cover Image (Featured Image)

The post cover is the most visible image in your Substack newsletter. It appears at the top of every post, as a thumbnail in the Substack feed, in social previews when readers share the link, and as the og:image for the web version. Choose a compelling image — it directly affects open rates.

Recommended size: 1000×563 px (16:9 aspect ratio)
Minimum size: 800×450 px — below this, images look soft in social previews
Format: JPEG for photos, PNG for graphics or illustrations
Target file size: under 200 KB for fast web loading
Keep important content centered — Substack crops the cover for different feed card sizes
Substack uses this image as the og:image for social sharing

Use the image resizer to scale your cover to exactly 1000×563 px, then use the image compressor to bring it under 200 KB before uploading.

Inline Images (Within Newsletter Body)

Images placed inside the body of a Substack post render at up to 800 px wide in both the web and email versions. Uploading at 1600 px provides crisp retina display on high-DPI screens while Substack scales the displayed size to 800 px.

Upload at 1600 px wide — Substack displays at 800 px (retina-ready)
Target file size: under 150 KB per image — this is especially important for email delivery
For email-only optimization, keep each inline image under 100 KB to ensure fast rendering on slow mobile connections
JPEG at quality 80–85 for photos — sharp enough, significantly smaller than PNG
PNG for screenshots, diagrams, and images with text — preserves crisp edges

Use the image compressor to reduce inline images to under 150 KB. If you use many images per post, aim for under 100 KB each to keep the total email size well within Gmail's rendering threshold.

Important: Email Image Delivery

Substack hosts all images on its CDN, so images themselves are loaded remotely rather than embedded inline in the email. This means image file sizes do not directly inflate the raw email size byte-for-byte. However, there are still important reasons to keep images lean:

Load time in email: email clients load images from the CDN when the email is opened — large files cause visible delays, especially on mobile
Images blocked by default: many corporate email clients block images by default; alt text remains visible but layout suffers if images are enormous placeholders
Bandwidth for subscribers: readers on metered connections benefit from smaller images
Keep each inline image under 150 KB; under 100 KB is ideal for newsletters with 3 or more inline images

Profile Photo

Your Substack profile photo appears next to your name on every post, on your publication homepage, and in subscriber email receipts. Substack displays it as a circle, so keep your subject centered and avoid placing important details near the corners.

Recommended size: 400×400 px (1:1 square)
PNG for illustrations and logos; JPEG for photos
The circular display crops the corners — center your face or logo in the middle of the frame

Use the image cropper to create a perfect square, then the image resizer to scale to 400×400 px.

Publication Logo

The publication logo appears in email headers, at the top of your Substack homepage, and next to your publication name in the Substack discovery feed. It is displayed at approximately 32 px in email headers, so keep it bold and simple.

Minimum size: 256×256 px (displayed at ~32 px in email headers)
PNG with transparent background preferred — renders cleanly on any background
Avoid thin lines or fine text — detail is lost at 32 px display size

Custom Header Image

If you use a custom header on your publication homepage, Substack displays it as a wide banner at the top of the page. This image does not appear in the email version.

Recommended size: 1500×500 px (3:1 ratio)
Keep essential content in the center — edges are cropped on mobile viewports
JPEG for photography, PNG for graphics with text or flat colors

Tips for Substack Writers

Cover images drive open rates — your post cover is what readers see in the feed and on social previews; invest in a high-quality, relevant image for every post
Compress inline images to under 100 KB each — for newsletters with multiple images, this ensures fast rendering in every email client
Send a test email to yourself — always check how images render in Gmail and Apple Mail before publishing; look for unexpected cropping, blurriness, or layout issues
Maintain a consistent visual style — using the same image dimensions, color palette, and style across posts builds a recognizable brand for your newsletter
GIFs are supported — Substack renders animated GIFs in both the web and email versions; keep them under 5 MB to avoid slow loading