Discord server banners: 960x540, 16:9, under 10MB. Those numbers come from Discord's own documentation, and this tool returns your image at exactly that spec.
Where the banner shows: behind the channel list at the top of your server, with the server name overlaid across the top of it. Keep the middle and lower parts of the image interesting and the top relatively calm, or the name lands on the busiest part of the picture.
Who can set one: server banners unlock at Boost level 2. If the upload option is missing, that's the reason rather than the file.
This produces a static banner. Animated banners exist at Boost level 3 and want a GIF, while this tool produces a sharp static JPG, the version every boosted server can use.
Bigger uploads work too: Discord accepts larger 16:9 images and scales them down. Uploading at the native 960x540 just means your own crop decides what survives, not their scaler.
The crop slider chooses which band of the source ends up in frame. Wide art and landscapes sit naturally in 16:9, while detailed emblems prefer the server icon instead.
Several images can go in together, each landing at 960x540, comfortably under Discord's 10MB allowance.