If you want to suggest "simple" methods to check if two files are identical before doing full MD5s, go ahead.

Thankyou. That's exactly what I already did. And all I did.

But...

Equally sized and colored Canadian and Chinese flags would have about a 50% chance of differing at a random byte (they are mostly the same shade of red). Similarly with United States and Japanese flags (they share a lot of white).

If these images are produced by a graphics programs, you (may, sometimes) be correct. If both images are produced by the same author, or both authors choose exactly the same shades of red or white. Maybe.

However, if these images are photographs, taken by different cameras, and/or different lighting conditions, and/or rippled by different winds, and/or catching reflected light from differently colored surroundings, and/or are different aged and therefore faded, and/or made of differing materials with respectively differing modulos of reflection, and/or the lens are dirty, and/or the cameras are differently focused, and/or compressed at differing ratios/quallities and/or dozens of other factors...

Your idealised judgement is wrong.