The ideal size for a banner cover image is 2112 x 640 px, to optimize for the largest size it can take. The minimum size for a badge cover image is 480 x 240 px.
The source image will be placed in the center and then zoomed until it covers both dimensions.
Removing and editing your cover
Once it's set, click on your cover to edit or remove it. Alternatively, choose "Edit layout" > "Edit cover image."