efficient way to compare 2 images?

[Kernelpanic]

If the variables are equal, the images are the same....

By variables do you now mean the same number of pixels? That would mean nothing.
Hundreds of thousands of pixels in picture 1 and 2 does not mean that they are identical, not even if it is the same color distribution.

Simple example:
10,000 Red - 15,000 Yellow - 20,000 White - 10,000 Green and X = 100,000 pixels. Even if the second image has the same number of colors, it does not mean that the images are the same.