Lex Jenkins , May 04, 2005; 01:43 a.m.
The best program I can think of is a pair of scissors and some experimentation.
I used to entertain myself and coworkers with my photocollages. We used cameras to document our field work and there would always be a few frames left over, so I'd use them up around the office. When I accumulated enough snapshots I'd cut and paste them together, using several photos of the same person to recreate them in collage form. It looked a bit like something from Picasso's Cubism era.
To make these experiments easier you might want to scan your photos and use cheap inkjet prints on ordinary paper.
If you want to create a collage with smoother joints and transitions you could use the cut 'n' paste method as a blueprint and then use scans of these same photos to make a finished version digitally.
Most decent image editing programs have tools for stitching and merging images together, but some are better than others. My versions of Corel Photo Paint and Jasc Paint Shop Pro are pretty old so there are probably more up to date utilities, either built in or as plug-ins.