Sorry, typo. Should have been "form" on when trying to edit a community. That's the only place and manner I saw as well. Maybe this is the type of thing that will grow out a nostr does.
Discussion
Is there a better source? My images from https://nostr.build/ look cheap and distorted. Most other community pics look crisp and clean?
You can use any source you want as far as I know for hosting images. Maybe upload to nostr as if you were going to include the image in a note, then copy/paste the URL into the banner slot on the form. I do a fair amount of stuff on the Hive blockchain, so I have several images housed there (hive.blog or peakd.com) and just use the URL that points to those servers. Maybe this helps.