efficient way to compare 2 images?

[Kernelpanic]

That's what I'm wondering. 
Maybe there's some magic "compare object" command that I don't know about. 
Evidently comparing it pixel-by-pixel, or a representative sampling of pixels, is a way to do it. 
But see my reply to Pete above to see why I'm asking this, there might be a better approach than comparing images...

Your image comparison is a core problem of the AI: Pattern recognition/comparison, or something. I don't know the exact name at the moment.
The problem was already described by Hubert L. Dreyfus in his first book in 1972: "What Computers Can't Do: The Limits of Artificial Intelligence".

This pattern recognition is essential for the survival of all living beings. For example: A deer must recognize at first glance whether what it sees is harmless or dangerous (such as a wolf).
All the best! Really! - Show the eggheads what basic programmers can do.