Discord server icon size and shape
Discord asks for a square image and recommends 512 x 512 pixels for server icons. Smaller uploads work down to 128 x 128, but they look soft on high-resolution screens, so exporting at 512 is the safer choice.
In the server list, every icon is drawn as a circle. When a server is selected or hovered, the circle rounds into a square with soft corners. That is why the tool shows both shapes: a design has to survive the circle crop, and still look right when the corners come back.
The same tool can export a 128 x 128 image for a custom server emoji, and a picture you can use as your own profile picture. Banners are a different shape (960 x 540 for a server banner), so make those in an image editor or a photo resizer instead.
Making an icon that still reads in the server list
Members see your icon most often in the narrow column on the left of the Discord app, stacked between every other server they have joined. At that size, detail disappears. One to three letters, a short abbreviation of the server name, or a single emoji are the shapes that stay readable.
Contrast does most of the work. A light letter on a dark background, or the other way round, holds up at small sizes and in both light and dark themes. The tool warns you when the text and background colours are too close to read. Heavy weights also survive shrinking better than thin ones.
A gradient background can make a plain letter icon look finished, but keep both colours similarly light or similarly dark, or the letter will be easy to read on one side and hard on the other.
Gaming server
GG on a dark background, heavy weight
Study group
📚 on a solid colour
Creator community
Your initials on a two-colour gradient
Uploading the icon to your server
Download the PNG, then open Server Settings and choose Overview. Click the current icon (or the upload area if the server has none), pick the file and save. Discord shows a crop step; because the PNG is already square, you can accept it as it is.
Only members with the Manage Server permission can change the icon. If the change does not show for other members straight away, it usually appears once their app refreshes.
If your server later reaches the first Server Boost level, you can replace the still icon with an animated GIF. Keep the still PNG anyway: it is what you will want for your server listing, social posts and anywhere else the community is promoted.