I agree with the above comment. 420x420 is almost a requirement on this kind of map, and just so you know, theese maps CAN be opened with the normal editor, but their porportions cannot be changed in it.
Also, i think you should add some tile variation to it, just to add more visual attractiveness to the minimap. Seeing how you only generated this from an image, that can hardly be too much to ask. Use Ashenvale Lumpy Grass and Ashenvale Grassy dirt for the majority, and then use some icecrown snow and maybe loarderon fall dirt for the desert.