I agree that there's no perfect comparison. I just think that whatever image format you use for the comparison, you should be storing an original picture, not one that has been stretched or otherwise modified.

FWIW, if you convert the 3KB image to 16 colours in GIMP (Image | Mode | Indexed... and choose "Generate Optimum Palette") it looks exactly the same. I'm pretty sure there are only 16 colours in the image. The resulting PNG is 1,991 bytes.