Well when you create the map, it's supposed to be 513 x 513
When you go to the map maker, you can change the height and such.
You can check this for some visuals / further help to see if you did this correctly.
Map Tutorial (http://www.feral-heart.com/index.php?option=com_content&view=article&id=6&Itemid=11)
No. What I'm saying, is...when you make the map in Gimp ( whatever art program, but Gimp is the best option so far ), you save the map as a 513 x 513.
Make sure your map is in greyscale though & you used the RGB when you 'colored'.