Hey Jan,
Thanks for contacting me. The issue you are seeing is unfortunately caused by two separate bugs in the current release version of Custom Maps. The actual cause of the "image too large" message is that the image name contains spaces or special characters. The second bug causes the wrong error message being shown.
The simplest way around this bug is to rename your map image file to use only letters A-Z, a-z, digits 0-9, and underscore and dash (_ and -). Note that this issue only affects the map image file names. You can use any letters you want in the actual map names that show up on your list of maps.
Sorry about the inconvenience. This bug will be fixed in the next release.
Author of Custom Maps