One of the advertising communities I'm a member of has a rule that if you post an image it has to fit within a width of 500 and a height of 300; can't be any bigger. Since the average icon is 100x100 and the current LJ setup makes it so that community icons spring up a menu that can direct you to join or friend an account...
...this seems like a really simple way to advertise 15 communities in one post!
Don't see why I can't do it with some others as well.
Kinda tempted to make them expanding icons, click and it takes you there as well, but I'm not gonna push my luck.