Well I had some ideas for you:
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Progress:
1) Your image (removed the tree coz it was distracting me)
2) Used Photoshop (cheating!
) to adjust the levels, then increased the saturation a bit.
3) Most ppl tend to avoid using random 1 pixel noise at this resolution. So I smoothed out the rocks and tried to give them some planar form (I suck at this >.<). Notice much stronger shadow on below grass.
4) A style of grass that seems popular these days. The basic principal from what I can see is to make the blades of grass larger than life, such that you can draw each one at this resolution. The result is a toony look you may not like but its handy to know you can do something similar with all sorts of textures for a smoother finish, rather than high contrast 1 pixel noise. There are plenty of other styles though, this is just one idea.
5) Started a small section of more defined rock shapes, more for my own practice than anything else. You may not like this kind of rocks or find it easier to stick with rounded shapes for now, and there's nothing wrong with that.
6) Tweaked colours and time for some highlights. TRYING to be selective with the highlights is always a challenge for me >.<
7) Some rough attempt at blending rocks into the darkness that is the middle tiles, reducing highlights along the way.
Much of the changes is targeted at communicating better to the player.
For example, squint your eyes and/or look NEAR the images but not directly at them, now notice how hard your platform is to see compared to my edit?
The bright yellow-green is a distinctive line that is visible in the heat of action without having to move your eyes all over the screen all the time.
To a lesser degree (intentionally), the highlights on the rocks also help mark the edge of the playable area.
Communicating in this way helps create a sense of trust with the player, and improves gameplay.
Sprites of course should take higher priority.
Using a chunk of unique, stand-out colour on your hero is a good idea to differentiate from all the enemy sprites (notice the yellow hair in Iconoclasts?).