Slack displays inline image previews up to 360 pixels wide in standard conversations and wider in threads. For optimal Slack display, images between 800-1200 pixels wide work best, as Slack will resize them to fit while maintaining quality.
For link preview images (unfurling), Slack uses the Open Graph image which should be 1200x628 pixels for the best display.
With ImgTools, resize your images to 1200px wide for general Slack sharing. This provides good quality when Slack scales the image down for display. For screenshots and documentation shared in Slack, 800px wide is usually sufficient.
Compress to JPEG at 80% quality to keep file sizes reasonable for fast loading in Slack. Slack has a 5MB file upload limit for free workspaces and higher limits for paid plans.
All processing happens locally in your browser.