I've been a long time member (with no gallery) at Deviant Art and recently I was about to start a gallery.  This led tot he reading of their TOS.  They claim rights to whatever you post for their own promotional and publication.

If they decide they want to sell mouse pads with your pic on it, they have that covered in their TOS.

At least this was my understanding.  I say avoid them like the plague or only upload low rez pics.

In the end, if you do cool, good stuff, you'll eventually see it stolen and posted elsewhere, no matter where you post (I've found stuff of mine that was only ever posted and Renderosity).  That's one thing.  The real thing I have issue with is someone trying to make money off of what I do.  If I can't, then sure as hell no one else should!!

I don't know the technicalities of MySpace, but again, easy to steal from.

In the end, though, the real question is why do you want to post?  If you want to gather interest in your work or hopefully attract clients, then MySpace and Deviant Art are almost required!

Still, if you do, watermark and lowrez it!

(This is my advice, I am not a lawyer or trained professional, but I could play one on TV if the money was good, the director was patient and I didn't have to wear a funny costume.)