Native USB is really hard to work (some would say almost impossible) with and even harder if you don't have the datasheet for the USB interface of the camera.
If you did pull it off, the camera is likely too fast while the memory card is too slow. The PIC isn't going to have enough buffer memory to bridge the two together. If you are just trying to store photos, there are RS-232 JPEG compression snapshot cameras available which are way easier to work with than USB. THey are only 1-6 FPS, but you with a slow, low memory PIC (and difficulty of working with USB) you aren't going to have much luck anyways making full use of the USB camera you do have. If you don't have the datasheet for the USB protocal you don't really stand a chance at all.
You will probably have way way better luck using one of these:
**broken link removed**