.jpg works as well, just type mapnamehere.jpg on the world tab, so .jpg or .png doesn't matter. What does matter tho, is size and such as said above. When saving a .png out of GIMP, you must be sure to pick the "Flatten Image" option, instead of the "Merge Visible Layers" option. Not choosing the right one can and will cause the game to crash almost instantly after entering the map's name. With .jpg, it will flatten the image for you as .jpg does not support layers like .png does. However, if your game continues to be dysfunctional after following thru with these steps and any other advice given, it could be that the file itself is corrupt and is best to delete it, and start again. Deleting the .png map file won't do much, as sometimes the GIMP file (.xcf) can become corrupt as well.