Some observations on designing a logo for Lemmy communities: during my working life (40+ yrs.) as a graphic designer, I worked on many logos. When designing a logo for Lemmy, keep in mind that the logo must work in a very tiny space, as well as larger applications. This means it cannot be overly complex. Also, it seems the default configuration for Lemmy logos is a round shape. So make sure your logo configures to this aspect. Finally, I'd go with something colorful and bright and eye catching. That's my 2¢, your mileage may vary.
We have a few ideas already for profile pics, but if you want to either add a new spin to one of them or maybe try putting together a community banner that would be awesome! I think the banner dimensions are something like 960x240, but can change depending on screens and user settings.
Can we put a banner up as well? I nominate this beach volleyball pic - I think it would crop well, and set the vibe this this is a chill place to hang out. https://lemmy.world/post/6371200