Do any good libraries and/or tools exist for automatic image "merging"?

Question

I am looking for a tool or library which will allow me to, very quickly, merge two source images, and generate a third, merged image, into a file.

My source data are two images in various formats (jpg, png, pdf, etc..), and instructions which identify which portions of the images are to overlap/merge.

By "merging" I mean any kind of transition effect from one image into the other - anywhere from, at the very simple end, an overlap with one image on top, to funkier things, such as automatic blending of colors and shapes.

I don't have a preference in terms of language and/or platform.

Answer 1

I used a DLL called CoolWaterMark with pretty good success.

Answer 2

ImageMagick's various composition effects can be used for this.

Answer 3

with ImageMagick, you can also use the montage command line tool to merge images.