Wow, I just made an awesome tree sprite from scratch in less than an hour:
It usually takes me several days to be satisfied with a new sprite, but that tree is absolutely beautiful! I didn't even use a reference image, I just started drawing at the base and by the time I got to shading it was already looking professional.
EDIT:The top clump is bushy whereas the other clumps are sharp, but other than that I can't see anything to fix.