The only pre-requisite for joining is that you have web space of your own to post images (or links to images) for use in this community. This is a visual community so I'd rather not see a lot of words but words can be visual too. If you're intending to post words, make sure they are also striking visually or descriptive enough to invoke pictures in the viewer's imagination.
I'm going to announce this community in my personal journal and a few other places before I begin posting visuals too but I will be taking an active role in this game since it's something I really like the idea of. I've played word-based free association for years but visual free association will be a new experience for me.
So, have fun. Post visuals. Post responses to other people's visuals. Post responses to responses. Layer and layer this thing full of imagery!
--Catalyst Echo (microchaotic)