Compress Images for Substack — Free, Fast & Private
Substack publishes the dimensions it wants for a publication’s design elements, and they are worth having in one place:
- Logo — at least 256 × 256 px, transparent background.
- Email banner — recommended 1100 × 220 px, transparent background, taller is allowed.
- Cover image — at least 600 × 600 px.
- Social / post preview image — at least 1200 × 630 px, at a 14:10 aspect ratio.
- Full-width images — maximum aspect ratio of 1:1 for height. Anything taller is automatically cropped square.
The 1:1 crop rule is the one that will surprise you
Insert a tall portrait photograph as a full-width image and Substack crops it to a square. Not scaled — cropped. If the subject sits low in the frame, the bottom of it disappears, and you will only notice after the post is out.
So decide the crop yourself. Bring tall images to square or landscape before uploading, positioning the subject deliberately. A 4:5 Instagram-style portrait is taller than 1:1 and will be cut.
Your post is an email, and email is not the web
A web page loads images lazily as the reader scrolls. An email does not: mail clients pull the whole message, images and all, often over a mobile connection, and some readers are still on metered data. A post carrying fifteen uncompressed photographs is a heavy email — slow to render in Gmail, slower still in Outlook, and clipped by Gmail once the message grows past its size threshold, which hides everything after the cut behind a “View entire message” link.
That clipping is the real cost of oversized images on Substack. It is not a rejected upload, it is a truncated newsletter — including, frequently, your subscribe button and your sign-off.
Practical sizing
Substack renders body images at a modest column width and scales them down for the email. Uploading at around 1456 px wide gives you a crisp 2× image on high-DPI screens without carrying pointless pixels. JPEG at the Smart preset is the right default for photographs; PNG only for charts, screenshots and anything with transparency.
If a post is image-heavy, aim to keep individual body images to a few hundred kilobytes rather than a few megabytes. Ten images at 250 KB is a comfortable email. Ten at 3 MB is a clipped one.
MiniPx compresses in your browser, so unpublished drafts and pre-release images stay on your machine right up until you publish.
Related: drafting in Notion first · the same images in a presentation
That 1:1 rule is one of several places a platform silently decides your crop — the ratios worth memorising collects the rest.
Cross-posting the same images to social? Facebook has its own weight limits and its own dimensions per placement, both different from what an email client needs.