1. More contrast. As I can hardly see, arms have two colours, but it is impossible to make any difference between them. The orange shades are too bright as well. Search for some references for shading, pay attention to the contrast between lit and shaded surfaces, and you will be surprised how different they are. If the darker spot on his right leg is supposed to be a shadow of the right arm, you should point that out. And in that case, the area under the shoulder should be darker, too.
Set neutral background. Shift it more to the grey.
After you finish lighting and contrast, think about detalization. His crab-hands especially need it. 

2.
Most users (I think) use imgur.com.
This forum doubles the size of images each time you click them (images). So there is no need to enlarge them yourself. Always show the real size, 1:1. It is also easier to make edits with the original scale.
Good luck in exploring the pixel world. 

Why are the highlights different on each circle? Such thing you drew is just impossible even with multiple light sources.

The grass spoils the image`s readability. As the rope, it contrasts with the ambience too much. The tree is not that bad, but in the place where it splits into branches it is too flat so the branches seem to be situated in one plane. Add some shadows there.
That big grey things - are they headstones or just boulders? In addition, they lie on one line. Try moving the middle one a bit left and up.

http://wouterpleizier.nl/pj/index.php?input=http%3A%2F%2Ffc09.deviantart.net%2Ffs70%2Ff%2F2014%2F078%2Ff%2F4%2F000_by_ffman22-d7at6ml.png
1362 colours. In one picture. You know, I`m not sure you did it as a pixel art piece. If you did it yourself, tell us which instruments you used. A brush, perhaps? You shouldn`t use instruments which mix colours in any ways. This looks nice, but it`s not a pixel art.