jwz has hacked a script to tile images in Perl using PerlMagick. Looks nice.

But as he said, he has hardtime generating a multi-layered image to be able to reposition pictures individual as the union of the set of file format the support this and the set if file formats that ImageMagick and the GIMP both work with seems to ne empty. ImageMagick does not write GIMP .xcf files, and GIMP poorly support .psd, Photoshop native file format. And GIMP Perl scripting seems to be... unusable (last I tried it, I didn't find proper documentation to understand how it works).

Maybe SVG compositing would help ?

I'll look at this, I like the idea.