1. try to save your images in png format , jpeg makes it look worse
2. why are you using over 100 colors on the left grass things on the wall? (at least my program tells me that there are over 100 colors lol)
Now actual critic:
The grass/moss on the left wall is way to noisy (you can't actually see what it should be)
The left wall needs some anti-aliasing
What are those red things on the tree?
Here is a edit, although it's not good at all (17 colors left):
I tried to remove the noise, but my time ran out haha
I tried to change the wood color but I don't think these colors fit in,so just ignore them (I wish I would remember where I got those)
I wasn't sure what those red things were, so I lowered the intensity and made them 2x2 pixels wide.
Good work for your first tries tho!
I hope this helps