Shadows and shading.  Since the building is casting a shadow on the wall behind it, the roof of the building would be shadowed, darker, that the other sides of the building.  also, the right wall should be slightly darker than the front wall since its not in the direction that the light seems to be coming from.  Also, what editing software do you use?  MSPaint?  Nothings wrong with bmp's, but later on you might want to consider using GIF format and MSPaint doesn't do GIFs very good.  good consistant iso.