Well if you're just looking to manipulate a photo, you could just find a photo on google that you like for the bite. Find something that matches your skin tone, and then save it to your computer. Open it in gimp along with the photo you want to manipulate, and then change the photo sizes accordingly so that the bite doesn't cover your face. Use the free select tool and zoom in to work around the area you want in your other photo. You might have to do this in a few steps, depending on the details you want to extract. Copy and paste the area to the photo you're manipulating and make sure the image is floating so you can move it. Anchor it when you're satisfied. Repeat until you have all of the details you want to move to your photo, good luck. By the way, if you're unsure of tools in gimp or how to create a floating image, anything along those lines, there should be tutorials all over the place. Otherwise, let me know ^_^
I don't know about gimp, but I'd just get a set of longish vampire teeth, cut the tips off, have someone bite someone else with them just enough to indent the flesh, adjust the lights, and shoot. Should be effective. I don't know that I'd use much, if any, fake blood, since there would not be any bleeding to speak of until the teeth were withdrawn. The tamponing effect (pushing, in effect, a pair of tapered corks into the holes) would prevent it.