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.
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
Compress HEIC for Other Platforms
Compress HEIC for Substack — FAQ
Substack works best with images under 10,000KB. Recommended dimensions: 1200×675 for newsletter cover images. ImageSmith can compress HEIC images to any target size in seconds.
Substack primarily supports JPEG, PNG, GIF. Converting to JPEG is recommended for best compatibility.
Upload your HEIC image to ImageSmith, set a target size under 10,000KB, and compress. Email subscribers see compressed images — pre-optimize for email quality
JPEG is the most reliable format for Substack newsletters as it renders correctly in all email clients including Outlook. Keep images under 200KB each for fast email loading.