No, no, thank you, your comments were very helpful.
Although not immediately, I think I understand what you mean now. The program I use to make maps converts the image with the ending to .PNG. Whereas the program I use to create portraits, on the other hand, .png. Since I have Windows and no possibility to test the game on other systems - I didn't even know that.
Looks like I'll have to change the code in all the map widgets, adding .PNG there instead of .png. If I understand correctly, this should fix the problem on Linux.