if I were trying to do someting like this I would use an 8bit index map and use index 0 as a null. the background would be for normal size tiles while the forground would be for larger tiles. then I would add a clip frame that reperesents the view able portion of the map. now all you have to do is move the clipframe around within the map to change the view. on the forground the indexed position would be the top/left and all indexes underneath it would be 0's. your clip frame would have to be larger then the actual view so if a larger tile were half on it would still be displayed.