Well, if you're accessing the site directly (and not through something like
TOR), and the cops know you access the site, they only have to talk to your internet service provider to get your address from your IP.
But honestly if you're not a huge threat (not posting pics of huge rooms full of buds etc) and don't have a rep as a distributor, they probably won't bother. Your real-world actions matter more than your online actions.. usually.
Of course it's important to isolate your rollitup usage from your normal email and facebook and whatnot, don't let them share any information. This provides plausible deniability, EVEN IF it came from your own house/computer. You can claim someone used your computer as a gateway with a virus, or someone hacked your wireless and used your internet (one of the reasons I prefer WEP encryption.. Strong enough to keep out the kind of people who live around me, but weak enough that it still remains an excuse).
EXIF data from photos you post may contain GPS data of where the pic was taken. This is another possible route to be busted. Open pics in mspaint or a photo editor, save as, different file type usually removes EXIF data.
Also: There's a
Legal forum here.