Can't be bothered to write up my usual softened, tactful critique. I'm going to be blunt, like Salmoneous.
First off, MS Paint has got to go! Switch to either
Paint.NET or
The GIMP. Both are free, both are easy to learn and use, and both are far, far superior to MS Paint. The GIMP is much more powerful, but you might prefer Paint.NET if you have an exceptionally slow computer.
Problems:
1. The ground colors aren't chosen too badly, but the "X" pattern looks
very bad. Use more intricate patterns, or at least thicken the lines and make sure the pattern is the same on every tile (as is, the ground will look even worse than it already does if it's made into anything other than a 3x3 square).
2. The contrast in the background tiles is
extremely high, making it look very ugly. Background tiles need
lower contrast, not higher.
3. The tree leaves are single-color chunks made with the spraypaint tool. TREES ARE NOT THAT SIMPLE, INFIDEL. THOU SHALT SPEND MANY MINUTES ON THY TREES, CAREFULLY SHADING AND SHAPING THEM, SUCH AS TO MAKE THEM LOOKETH LESS LIKE CRAP.
4. The trunks on the trees at the top are all right (though they'd look better with some slight shading) but the trees at the bottom left have very oddly shaped, thoroughly unnatural-looking trunks.
5. The bright green grass is too thick and a little too bright in color.
6. The darker grass is too scribbly. It looks more like moss than grass.
7. The pillars are very ugly. Their bodies are too thin for their wide bases, and their edges are far too sharp; they need a bit more shading and detail, and also a duller color.
8. Looks like you've added two tiles that are an attempt at making semitransparent tiles. This is one of the biggest arguments against MS Paint; it has no transparency capability whatsoever. Again, switch to something else.
9. The windows in the background, in addition to being too contrasted to the background tiles and being way too bright in the first place, are oddly shaped. It is difficult to tell that they're supposed to be windows.
10. What's the little white star thing for?
11. The buds on the plants need to be more consistent, and probably need to be smaller, too.