I figured out what the problem with using trim.txt was (by reading the code).
All the files in the zip must be uncompressed.
The desc.txt file is small enough that it probably won't get compressed.
The trim.txt is a bit longer (and repetitive) and might get compressed (and ignored later).
Here's the same count up in the lower right corner, smaller, now that it uses 100x100 images.
trim.txt looks like:
Code:
100x100+932+1318
100x100+932+1318
100x100+932+1318
...
desc.txt looks like this (with background color):
Code:
1072 1448 1
p 0 0 part0 #ffffff