The problem is not one of pixelling, it is one of construction. The scene is too ordered. two rows of trees, aligned, same shape (mostly), shame tendrils, same leaves, and in the back, symmetrical shadows of more implied trees. The rocks the trees are one are the same too. What you should have done (and sadly it's probably too late to tweak at this stage) is use reference of real pictures of swamps and forests, if not directly, just so you can have the fractalized orderlesness of reality in mind when you draw.
Now, there is this interesting byproduct of this effect, that the scene seems more gameartish, like implying tiles and the like, but I both doubt you were going for this, and that it actually helps this piece.
This piece will not be as good as you need it to be without a major overhaul.