I downloaded a custom ROM for my Samsung Captivate phone that is a variant of 2.2.1. Its version of the Camera allows me to use the power button to take pictures. I decided I want the stock camera without this feature. Is there a site from which the default camera.apk is available for 2.2.1, or for that matter, other apps?
So I just looked at the firmware for my Samsung Vibrant, you don't actually need to mount the image -- I was thinking of the recovery image or a nandroid backup. The firmware (ROM) should be in a zip file, which you can open up and go to
/system/app/ and grab the camera.apk. I'd put it on your phone's SD card, then use Root Explorer or similar to copy it into
/system/app/ on your device and overwrite the current one (which you may want to back up first).
You can check out my answer to another question for info on mounting, but you shouldn't have to do that :)