Question: I have two different pictures of the same object, one of which is taken from roughy the same angle but has a different scale and rotation. I want both images to overlap so that the upper one matches the lower one as exactly as possible. Is there any option in gimp where I can define a number of key points and gimp aligns the images according to these points? For example I say that corner X in picture 1 has to match corner Y in picture 2. It should be sufficient to define 3 such points in each image to get a good result already.

If it is not possible, what would be the best way to reach this goal aside from just scaling and rotating until it looks ok?

Answer: 
I don't believe there is anything in Gimp to auto align images.

You can do it manually by putting the images in layers, setting the top opacity so you can see the underlying image and scaling/rotating one of them.

Or you can use a tool like [Enfuse](http://enblend.sourceforge.net/)
You can use Filter|G'MIC|Layers|Align Layers. You must select Input Layers: All before executing. Take into account that this is a time consuming operation.