Cover photos for profiles and communities have an aspect ratio or 3:1, so if you use that at a decent resolution, you should be good to go.
We recommend designing your cover image at a resolution of 3000 pixels wide x 1000 pixels high, then exporting as a JPG or PNG.
Stuck for ideas? Contact us, and we will see if our design team can create something cool for you!
Here are some examples of cover images made by our designers.