Have you ever used GML? this system is too simple to function efficiently. sorry. Anyway, My programming team is currently creating a fully 3D, terrain destructable, multiplayer, multi race, absolutly awesome Tremulous RTS, so work on your 2D system but please, use at least some basic GML. I have included some examples.
image_angle=direction (simple, but the front must be on the left side of the sprite)
if place_free(x,y,obj){insert commands}(checks whether the place specified is empty)
sprite_index=[sprite name] (changes the sprite)
image_speed=[number] (speed which the sprite's subimages alternate at)
once you fail this why not come support us on our thread? you might learn something interesting. And by the way, don't bother attempting anything difficult (like 3D drawing, it can be done on game maker) until you at least know the basics of GML.