The top image seems the most developed, but it's kind of confusing because you have a character as well as environments in the same image. I'd like to see that character developed more in a traditional "turnaround" form (show multiple views of the character). It look really interesting, but just seems out of place amidst all those trees.
I like the silhouettes of the trees, but - again - it would be nice to see more detail. Simple highlights and shading would help a LOT.
It's tough for me to critique the other images because they simply aren't far enough along yet.