Hey Cliipso,

first of all good work! And kudos for your will to improve that's the greatest skill an artist can have ;)

Regarding improvements for your tileset:

The light and shadow in your picture are to hard and look a bit unnatural.

I think it might look better if you go with a more thin line of dark and smoothly blend over to the brown sand color. The color for your highlighted areas is too cold and greyish. Maybe you could simply use a little more contrast to the original ground color?

Although the walking ground on here seems more like waves with up and down points so instead of using one straight top lightning how about going with the shape? Blending in with the ground color again would add up to the effect I guess. 

In case my english is too bad or weird ^^ here is a picture of what I just tried to say.

Hope that helps. Good work!