Make sure (on gimp/photoshop what ever you use) you have made your main map with the height and width as 513x513 and your settings set on Greyscale (It will only work on grayscale) Also make sure that you have saved your map as a .png file NOT .jpg or anything else.
(http://i48BannedImageSite/10qlblt.png)
Same with your map mask but as RGB colour
(http://i47BannedImageSite/2i8ei5x.png)