PR expects 0x28 (40) byte headers, but GIMP saves 0x6C (108) byte headers. (see here)
The first problem is that PR does not accept BMP files if the size of their headers are not 40 bytes.
The second problem is that PR does not show any error message when this happens.
Norbert, I just fixed both problems in my copy of PR.
(Did you make the images of your mod with GIMP? If you did, then did you meet this bug?)
Kamil, thanks for pointing it out.

Solution:
In GIMP, when you export to BMP, use the "Export... (Shift-Ctrl-E)" menu item, and not the "Export to whatever.bmp (Ctrl-E)" or "Overwrite whatever.bmp".
After you press Export, click "Replace". Then a small window will appear.
It contains a "[+] compatibility settings". Open it, and check the "don't write colorspace information" checkbox. Click "Export".