I have used ImageMagic to my thumbnails, and icons in my FVWM. It's a very cool toolkit, but not enough effective on my old PC. First was great to seeing the thumbs, but after five days.... I don't like waiting for windows, and the 3 or 4 sec was too long.
I have found a great topic in the FVWM forum. There is a great perl script, and a very useful C source ([link] thx for everyone ).
One day and 79 line after I have a cpp app. It's now enough good, 3 or 4 times faster than the xwd, convert combination (with big windows more effective), and the C source is only 0.005 - 0.010 sec faster. Now I'm satisfied  with my little prog.
Next time I want in place of composite something. After that? I don't know. A module, if I have time, and mood...