Changing the picture representing a map freezes my game

[Vaxelin]

Whenever I change the file that represents a map, it always freezes my game. I used GIMP to edit and the images and convert them to a PNG of the corresponding size to the image, so I'm not exactly sure what the problem is because I use the same method to change CSP's all the time. What could be causing this? Does the file need the exact same name? Does the common3_en.pac in my system folder have anything to do with it?

[Tsuruhime1029]

I'd start with having it be the exact name. It's possible that the image has to be a certain size, so I'd check the size of the game's image and copy that size. Also, what are you using to do this?