Dimensions

For summary_large_image unfurls: 1200 x 630px recommended, 300 x 157px minimum. Below the minimum, the image may not display at all.

For summary (thumbnail) unfurls: 512 x 512px recommended, 144 x 144px minimum.

Size and pixel limits

• Maximum file size: 1MB (larger images may time out at the proxy)
• Maximum longest side: 25,000px
• Maximum total pixels: 45 million (e.g., 6708 x 6708)

Images over these limits get silently dropped, and you just get a text-only unfurl with no error.

Formats

• JPEG: best for photos
• PNG: good for graphics with text or transparency
• GIF: animated GIFs play inline in the unfurl
• WebP: partial support; JPEG or PNG are safer

Image proxying

Slack fetches images through Slack-ImgProxy and re-serves them. The image URL must be publicly accessible at crawl time, and the server needs to respond within a few seconds because the proxy has tight timeouts.

URL requirements

The og:image value must be:

• An absolute URL (not relative)
• Accessible without auth
• Served over HTTPS
• Fast to respond (slow image servers cause silent failures)

Layout

Slack shows unfurl images below the text in a left-bordered card. The image scales to the card width, so landscape-oriented images work best. Keep key content away from the edges, since slight cropping can happen at different display sizes.