Aspect Ratios to Use for Slack Messages

Updated July 26, 2026 · 8 min read

Aspect ratio is the shape of the image, and the wrong one leads to awkward crops. Shared images live in workspace storage and get re-downloaded by every viewer, so smaller is kinder to everyone.

Recommended settings for Slack messages

SettingStarting point
FormatJPEG (WebP where accepted)
Widthabout 1600 px on the long edge
Quality78
File size500 KB or less

These are practical defaults. If the platform or form you are using publishes its own limits, follow those instead.

Aspect Ratios to Use

Common ratios are 1:1 square, 4:5 portrait, 3:2 classic photo and 16:9 widescreen. Pick the one that matches where the image will be displayed for Slack messages.

Crop to the ratio first, then resize to about 1600 pixels on the long edge and compress. Cropping last can undo your careful sizing.

Step by step with TheCompressJPEG

  1. Open the compress JPEG to 500 KB tool, or the main compressor for custom settings.
  2. Drop in your image. Nothing is uploaded; it is processed in your browser.
  3. Set the maximum width to 1600 and the target size to 500 KB.
  4. Press Compress, check the result and download it.
Ready to try it? Compress your JPEG to 500 KB now. It's free and private.

Quick tips

  • Write a size limit slightly below the stated maximum to leave a margin.
  • Process private images locally rather than uploading them to unknown servers.
  • Crop out distractions so pixels go to the subject.
  • Use WebP where it is supported and you control the page.

FAQ

Can I compress images for e-learning courses without uploading them?

Yes. TheCompressJPEG runs entirely in your browser, so your images never leave your device.

Is WebP accepted for e-learning courses?

It depends on the platform. JPEG is the safest choice when you are not sure.

What size should an image be for e-learning courses?

A practical default is about 1600 pixels wide and 200 KB or less, unless the destination states its own limits.

Wrapping up

For Slack messages, the reliable recipe is simple: resize to around 1600 pixels, compress at quality 78 or to a 500 KB target, and check the result before you upload. Keep the original so you can always start again.

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