Why YSK: Transparent background images often don’t look very nice on Reddit, but they can be quite sleek over here. For some examples just look at the profile pictures of mod list for this community on the sidebar. They also work well for communities. Be careful to pick colors that aren’t too dark or too light, and which contrast well with your community’s banner.
I think it has to do with the fact that it’s a grayish dark background as opposed to the bright white or pitch black that you normally get with Reddit. Our community icon certainly looks amazing here in Lemmy.