I'm not familiar with these communities, but you can also cross post this to !fedigrow@lemm.ee. That community is good about discussing which community to post in, and maybe organize some collective effort to lock a few of them to redirect efforts to a particular one.

Once you decide on one, it might also help to do the following:

1. Post in the selected community
2. Crosspost to larger spaces like !pics@lemmy.world
3. Leave a comment in the crosspost linking back to the community

That way people can discover the community over time

They're not that active, but theres also !wrzuty@szmer.info, !graffiti@sh.itjust.works, !vangraffiti@lemmings.world (if that counts), and !freights@lemmy.world.

There's lots of activity on the rest of the fediverse under hashtags like #streetart and #graffiti, and accounts like RadicalGraffiti and StreetArtUtopia.

I didn't even know we had communities for this. I'll remember next time I walk past graffiti, there's some interesting ones where I live but no, I don't have a collection of photos.

The way to get more activity is to just post. You just happened to speak up, but in all likelihood more people saw there wasn't much activity and completely forgot about it. Lemmy is relatively small, so it makes sense that niche communities don't get as many posts.