Well I guess !climate@slrpnk.net !energy@slrpnk.net and !technology@slrpnk.net are already quite well known.
!balconygardening@slrpnk.net and !inperson@slrpnk.net are kinda cool as well. In general have a look at our community list, we have some good ones that are at least reasonably active.
I changed the pict-rs settings a bit to allow more file types, but I assume your main issue is frontend and/or browser related.