Suppose I have two images, such as

and

and I want to stack them vertically like this

What's the easiest way to do it?
What I do now is:
1) Copy paste image 2 onto image 1 as a layer
2) Layer->Change Layer Size on background to match combined size
3) Position image 2 in proper place
4) Layer->Reduce to Background
Since I have over a hundred of these, a batch process or something nearly automatic would be best...
Thanks!