Compress HEIC Images for Substack
Substack newsletter images are embedded in email and displayed on web. Images must be optimized for both email clients and web browsers, with email-safe formats prioritized. High Efficiency Image Format — the default photo format on iPhones and modern Apple devices. Excellent compression. Use ImageSmith to compress your HEIC images to the ideal file size for Substack — target under 10,000KB for optimal performance. Processing is fully browser-based — no server uploads, no privacy risk.
Recommended format
JPEG
Max file size
10,000KB
Dimensions
1200×675 for newsletter cover images
✓Email subscribers see compressed images — pre-optimize for email quality
✓Keep email-embedded images under 200KB for reliable delivery
✓Use JPEG for photographs — Substack email clients do not support WebP
✓Cover images display at 1200×675 in web and email previews