Log-Polar DFT Based Scale-Invariant Image Registration
If you want something really robust, but that might be more computationally expensive, you might want to check out the algorithm I implemented here. It implements the paper, "Robust Image Registration Using Log-Polar Transform" (pdf). It also has the advantage of being rotation-invariant, in addition to translation and scale invariant. In my application (art), it was able to register even even similar-looking images, not just transformed versions of the same image.