Well if you want you can just be non-specific about it:
img { image-rendering: -moz-crisp-edges; -ms-interpolation-mode: nearest-neighbor; }
It will apply nearest-neighbor to all images that aren't at 100%. I'm probably just going to go with that, to simplify things, but those who may want to have it applied to jpegs and such might not like such a solution.
You can also do this:
img[src$=".gif"], img[src$=".GIF"], img[src$=".png"], img[src$=".PNG"] { image-rendering: -moz-crisp-edges; -ms-interpolation-mode: nearest-neighbor; }
To compress things a little.
You might also be able to pull something like this off:
img { image-rendering: -moz-crisp-edges; -ms-interpolation-mode: nearest-neighbor; }
img[src$=".jpg"], img[src$=".JPG"], img[src$=".jpeg"], img[src$=".JPEG"] { image-rendering: auto; -ms-interpolation-mode: bicubic; }