Slack Image Sizes — Profile Picture, Workspace Icon & Emoji

Slack workspace icon size — 512 x 512

The workspace icon is the tile in the sidebar rail that people click to switch workspaces. Slack accepts GIF, PNG, JPG, JPEG, HEIC, and HEIF, and walks you through a crop step on upload — so a non-square source will not be rejected, it will just be cropped for you.

• Upload 512x512: Slack does not publish a minimum, but 512x512 is the same square it asks for on profile photos and it leaves headroom for the retina rail tile.
• Crop it yourself first: Slack’s in-app cropper works, but cropping in your editor is the only way to control what gets cut. Upload an already-square file.
• Ship the mark, not the logo: the rail tile renders at roughly the size of a favicon. A wordmark with a tagline is illegible there; an icon-only lockup is not.
• Corners get rounded: Slack draws the icon as a rounded square. Keep anything load-bearing away from the corners.
• PNG for flat marks, JPG for photos: the same rule as everywhere else. A transparent PNG composites cleanly against both the light and dark sidebar.

The same 512x512 master covers a Discord server icon and a Telegram channel photo — build it once and upload it three times.

Slack profile picture size — 512 x 512 min, 1024 x 1024 max

This is the one spec Slack states as an exact range. The users.setPhoto reference puts it plainly: please limit your images to a maximum size of 1024 by 1024 pixels. 512x512 pixels is the minimum. Both ends are real. Undershoot and it looks soft on a retina screen; overshoot and the API returns too_large — the uploaded image had excessive dimensions.

• 512x512 minimum, 1024x1024 maximum: anywhere in that square range is fine. 1024x1024 is the safest choice if you want the crispest render on high-DPI displays.
• Square, or Slack crops it: the photo is displayed as a rounded square next to every message you post.
• JPEG, PNG, or GIF: anything else fails with bad_image.
• Animated GIFs have a frame limit too: the API rejects them with too_many_frames. A still photo is the safe path.
• Crop tight: the avatar renders at around 36 pixels in the message list. Head and shoulders, not a full-body shot.

Slack custom emoji size — 128 x 128, under 128KB

Slack is unusually terse about emoji specs. The help centre says only that square images under 128KB and with transparent backgrounds work best, that images can be JPG, PNG, or GIF, and that GIFs can include up to 50 frames. There is no published pixel maximum — but there is an enforced one.

• 128x128 is the target: big enough that Slack never has to upscale, small enough that a PNG stays well under the 128KB ceiling.
• 128KB is the real constraint: it bites on animated GIFs long before it bites on static PNGs. A 50-frame GIF at 128x128 will blow the budget unless you cut frames or reduce the palette.
• 50 frames, hard stop: Slack documents the limit explicitly. Trim the loop rather than dropping the frame rate to nothing.
• Transparency, always: emoji sit inline in messages and inside reaction pills, on white and on dark. A baked-in white background looks like a sticker on both.
• Oversized uploads are resized, not rejected: Slack downsizes them for you. The API surfaces the underlying limit as error_bad_wide and resized_but_still_too_large — so a 2000px source that compresses badly can still fail.
• Design for 22 pixels: the emoji renders inline at roughly text height. Fine detail, thin outlines, and any lettering disappear. One shape, one silhouette, high contrast.

Emoji are where Slack and its neighbours diverge — a Discord custom emoji is 128x128 under 256KB, and a Telegram sticker is 512x512 with one dimension exactly 512. Re-export per platform rather than reusing one file.

Slack app icon size — 512 x 512

The app icon is what you upload under Display Informationin your app settings: the avatar your bot posts under, the tile in the Apps list, and the logo on your Slack Marketplace listing. Slack’s design guidance names two numbers rather than an upload limit — pick a compelling logo that will look great, adaptively rendering at 512x512px on desktop, and 36x36px on mobile. Design the 512, but check it at 36.

• Don’t round the corners: Slack’s docs are explicit — “Slack will do it for you.” Pre-rounded corners get rounded twice and the mark ends up floating in a gap.
• Illustrations beat photos: a flat, high-contrast mark survives the drop to 36px in a way a photograph does not.
• No paragraphs of text: Slack warns against text that will be hard to read at a small size. One or two letters at most.
• Resolution is a review criterion: the Marketplace guidelines ask for an icon of high enough quality that it isn’t blurry in the listing — a 512x512 export from vector art clears that bar, an upscaled 128px favicon does not.
• Listing screenshots are a separate asset: 1600x1000 at 8:5, under 2MB, in JPG, JPEG, or PNG — and Slack asks that they show your app inside Slack, not inside your own product.
• Reuse the workspace icon square: if your app represents the same brand as the workspace, one 512x512 master covers both slots.

Slack message image upload limits — 1GB, 360px inline

You can upload a file up to 1GB to Slack — but the image everyone actually sees is much smaller. Slack generates a fixed ladder of thumbnails for every uploaded image and renders the 360px one inline in the message. That is why your 2560px desktop screenshot arrives unreadable.

• Crop, don’t shrink: a full-screen 2560px screenshot squeezed into 360px turns 12px UI text into mush. Crop to the region you are actually talking about and it survives the downscale.
• 1024px on the longest side is enough: that is the largest thumbnail Slack generates. Uploading a 4000px original buys nothing but upload time.
• 1GB per file: the ceiling applies to anything you add to a channel or DM, not just images.
• Two ceilings kill the preview entirely: Slack states an image only previews inline if it is smaller than 25,000 pixels on the longest side and under 45 million pixels in total. A stitched panorama or a full-page screenshot export can pass the first test and fail the second — it uploads fine, it just arrives as a file card with no picture.
• Ten files at a time: Slack accepts up to 10 files dragged into the message field in one go.

Slack link preview image — 1200 x 630 og:image

When someone pastes your URL into a channel, Slack builds the card itself: it crawls the URL, looks for common OpenGraph and X Card metadata, and renders some micro-approximation of the content. Slack publishes no dimension for that image, so the spec to hit is the standard Open Graph card — 1200x630 at 1.91:1, the same master you already ship for LinkedIn and X. Get that tag right and Slack, LinkedIn, and X are all covered by one file.

Slack lists the cases where a preview will not expand at all — worth checking before you blame the image:

• The page has no preview data: no OpenGraph or card tags, no card. This is the usual cause.
• The link is private or password-protected: Slack’s crawler has to be able to reach the page anonymously, so a staging URL behind auth will never unfurl.
• The same link was posted in that conversation within the past hour: Slack suppresses the repeat, which looks exactly like a broken card.
• The URL is missing the http:// protocol: a bare domain is not treated as a link to crawl.
• The message contains more than five links: Slack stops previewing them.
• Posting as an app instead? Attachment images are not the same as an unfurl. image_url is capped at 360px wide or 500px tall and thumb_url is scaled to 75px, so send a small, already-cropped image rather than a full card.

Resize an image to 1200x630 → Pick the “Link Preview (og:image)” preset in the resizer above.