I never hoped Arduino can process the images themselves. The reason I mentioned 'uncompressed' is the size of the higher resolution images'. A normal 640*480 JPEG image has only 120K bytes, however a 16-bit color bitmap has more than 600K bytes. Even though we could read images from a FIFO camera module with Arduino, we couldn't reach a reasonable time due to its uncompressed size.
So the best solution I thought is to get JPEG images from a FIFO camera with SPI bus. The next step is to wait for the hardware a guy promised me to do in two weeks. I'll tell you the progress of it later.