Batch merging separate image+alpha files

[Timpe]

Hi. I have a bunch of images where the black&white alpha mask (i.e. the transparency data) is in a seperate file from the actual image. How would I go about easily batch merging them?

http://abload.de/img/10pjay.jpg
^not my image, but the same situation. e.g. **.png and the matching **_alpha.png need to be merged into a single **.bmp where the latter image is the alpha channel. Even better if it goes straight to one transparent png.

Any help would be much appreciated.