What can you do if someone uses your graphics w/o permission? Said he takes no credit for the thing but slaps on his logo on top of it. Can you do anything?

Not much. You can try to get them to take it down, but people who take stuff are stubborn bitches. You can try suing for copyright infringement if you made everything in the layout (images, brushes...everything). I don't really think you want to pay for a lawyer on something so minor, though. You could start putting a big ol' watermark on your graphics from now on to prevent it. That's what I ended up doing after three ginormous myspace layout websites took my divs...

You could report him to his host. A lot of times they'll take care of it by shutting down the person's site.

Firstly, I would try to be civil with him/her. Ask him to remove it and tell him that it's yours. Sometimes people will be like "Uh oh, I got caught" and don't want to cause any drama. If they want to complain to you about it, then tell them that it is copywritten and you will do something about it. If that still doesn't work, then, yeah, contact their host.

Moved to Webmasters forum.

maybe you should start adding a watermark to your graphics...

IMO, it depends on what it is. if it's something you've worked REALLY hard on, i would (as already mentioned), first ask for them to remove it, then threaten with legal crap, and so on...
however, if it's something like a simple icon or image, i would ask for credit, but if they don't, i let it go. sometimes it's not worth the drama.
for example, a while back i found a user here on CB who started using the same image as their profile display picture when i started using it as my forum avatar. i asked for credit, but she claimed she got it from one of her friends.
i was so close to digging through my pile of disks to find the PSD for that icon, but i decided it wasn't worth it. i had other things to spend my time on.

catch my drift? that's just my opinion/experience though.
good luck. :]

i'm going with this answer. if you calmly contact them and let them know you know they're doing something they shouldn't, they'll more than likely stop. i just had someone take my site layout (verbatim coding and everything), and after i mentioned to them i'd appreciate it if they took it down, they did within a couple hours.