Jad is absolutley right bloodblade 100%.
Reading between the lines and based on how you've described your restrictions - "20x20 tiles (to those who cared)", " 32x32's (or sumthing like that)" makes me think that you've not got a realistic handle of what it is you're attempting to do. Coupled with the fact that what I'm looking at is what i'd refer to as "Naive" art... (sky is blue, grass is green, tree trunks are brown, rock is grey, &t &t) and the fact you've said yourself you're no expert... now some of us around here are quite adept at pushing pixels and equally adept at criticizing and commenting on each others work... this is how we roll... this is how we learn... this is how we grow as artists... or should I say, this is how we SHOULD grow as artists.
Learning to take criticism and apply it accordingly is part of that learning curve and shows true proffesionalism when one can take it without feeling the need to react negatively. It's a bitter pill that one must learn to enjoy if one wants to improve.
You've taken steps already on this path of artistic enlightenment by criticising your own work... your trees don't suck... it could be a stylistic choice... don't be too hard on yourself... but if they don't measure up to your expectations of what's in you mindseye then do it again, until it's right, and study other peoples work to get inspiration... I know its corny but its so true that learning comes from mistakes...
But enough of that... on to the piece in question.
I for one am a little perturbed by the notion that the game goes from extremely low detail to extremely high... speaking with my producer hat on I'd have to ask you, if this is your example of extremely low detail then please illustrate extremely high, then one can make an assessment of the range of difference between the two extremes and perhaps, in context, be able to C&C a little more accurately.
If I were you I'd start like this, rather than trying to tackle too much at once... just focus on a couple of elements at first, say the rock and tree... and do a few iterations of them using this lo detail through to high detail idea.
I'll look forward to seeing what you come up with.