An example of a "photographic" image:

This is an original image, a JPG saved at the "10" setting (effectively no compression) file size - 34KB

This is the same image, still a JPG saved at the "3" setting - it still looks very good - file size is down to 12KB

This is the same image, saved as a 5 bit GIF. The most obvious problem is the loss of color information on the tank - file size, 11KB