When scanning any type of map you need to save the map in either a .jpg, .tif, .bmp or .pcx format. This is actually done by the software that is used for scanning the maps. If the map is in a format other than one of these, you will get an error message that the map is in the wrong format.
Another possible causes for this problem is if the file containing the scanned map has become corrupted.
In either case it is best to rescan the maps and try loading them again into Farm Works.