Resize an image for Slack on Mac

You drag a screenshot into Slack and it shows up as a 4 MB blob with a slow-loading preview that everyone in the channel waits two seconds to see. Slack’s hard upload cap is 1 GB per file — that’s never the problem. The real problem is that anything over a few MB stalls previews, eats free-tier storage, and looks lazy.

Why this happens

Slack auto-generates a low-resolution preview thumbnail in the channel, then loads the full-res file when someone clicks. Above about 2-3 MB, the thumbnail generation starts visibly lagging on most networks. On free workspaces, total team upload storage is capped — every oversized image counts against it.

Also, the actual display size in a Slack channel is around 360px wide for the inline preview. You’re often uploading a 3000px-wide image to be displayed at one-eighth that.

The native way

For one image:

• Preview. Open it, Tools → Adjust Size, set the width to 1600 or 2000px, then File → Export and drag Quality to about 75%. Save and drag into Slack.

• sips from Terminal. Faster if you know what you want:

  sips -s format jpeg -s formatOptions 75 -Z 1800 screenshot.png —out slack.jpg

  Note the format change — PNG screenshots are huge; converting to JPEG cuts another 60-80% on top of the resize.

The faster way

For more than one or two, Download Melt. Drag in a folder, set output to JPEG at quality 80 with a 2000px cap, click Compress.

1. Download Melt and open it.
2. Drag the screenshots or photos in.
3. Set output format to JPEG (smaller for photos and screenshots both) at quality 80.
4. Enable Resize and cap the longest edge at 1800-2000px.
5. Click Compress, then drag the output into Slack.

How much smaller will it really get?

A typical 4032x3024 iPhone screenshot weighs in around 4-6 MB as PNG. After Melt: 250-500 KB as JPEG. A burst of UI screenshots from a design review goes from 60 MB total to 4-5 MB — and Slack previews them instantly.

The real reason this matters is that nobody on a phone wants to wait three seconds for a screenshot to load. Smaller images aren’t just polite — they’re faster to look at.