Compress Images Before Uploading to Slack
Overview
Slack previews images in-channel but generates thumbnails from the original file. Large originals (5-10MB) cause slow previews for team members on mobile connections. Pre-compressing to under 1MB makes thumbnails load instantly without affecting the visible preview quality.
How to Use This Tool
Upload your image. Set quality to 80% and width to 1200px. Click Compress. The output is under 500KB for most photos. Upload the compressed image to Slack. Previews will load faster for the whole channel.
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PDF Tool →Frequently Asked Questions
What is the maximum file size for Slack uploads?
Slack allows file uploads up to 1GB per file. However, for images, staying under 1MB ensures fast preview rendering for all team members including those on mobile connections.
Does Slack compress images automatically?
Slack generates thumbnails and previews from uploaded images but keeps the original file available for download. Pre-compressing gives team members faster previews without waiting for Slack's thumbnail generation.
Can I send images larger than 1GB on Slack?
Slack's limit is 1GB. For files above 1GB, use external storage (Google Drive, Dropbox) and share the link in Slack instead of uploading the file directly.