grass.png - Verrry nice. The table thing is a little odd, but not badly tiled. My only complaint is that the grass is very plain... I know most people hate doing it, but for good-looking grass, it needs to look natural, and that means weeds, tufts of grass, the occasional flower, etc.
snow1.png - Decent. The snow doesn't have much detail, but that's snow for ya =/ The transition between the vertical and horizontal cliff face might have been better if you'd made the vertical one turn diagonally and then horizontal, thus connecting with the horizontal cliff face.
snow2.png - Very good! Once again the snow doesn't have much detail but doesn't need it. Strangely, although this level is kinda plain, I think adding much more might make it feel a little crowded.
castleway.png - Simple, yet freakin' awesome! Of course, people without graphics cards might not feel the same way
sanctuary.png - It's too bare. The wooden tiles on the walls are a nice touch. The carpet is kinda overused, but it has a nice fringe.
Keep up the good work!