efficient way to compare 2 images?

[Pete]

I think we are at the point of asking exactly what images are to be compared? If I were using/making my own, they would all have the consistency to be compared as apples to apples. No one saved via compression vs the identical one being saved in the standard format. But what about a jpeg that is saved at high resolution vs low? Exact same image, but less definition. So is it the same or not here?

Many times to get the solution for the situation requires much more involved info.

Pete