Lord of the memes

Bear in mind most apps have only been in development for a couple of weeks at most and they're all prioritising different features. You can always log into the mobile site and upload an avatar that way if you can't yet do it in whichever app you've settled on.

(For your other question, as claymedia explained lemmy.world and lemmy.ml are just two instances of many that are both running the Lemmy software, with different signup rules and different communities hosted there. But since you can access the communities from wherever, just pick whichever home instance you like and get subscribing)