I know this post is old, but there weren't any replies...so..
First off, it's very good work. You are at a level that most people never get to. The part of your work that shines the most is your (generally) perfect use of focus.
There are things that stand out to me: On many of the backgrounds, you seem to under-expose them quite a bit...this problem is compounded by the pattern overlays that just serve to darken and blend the compositions. It's like an itch I can't scratch. I like the designs, but they look bland and dark.
Ok, the other thing. Your photmanipulations are good. You have a very good grasp of render placement, saturation(with the exception of the dino one...it looks jersey shore tanned), and color matching...Basically, you can make things look like they belong in the PICTURE, but not in the place. I think you need to practice using details to place the renders in the setting, rather than blanket effects. The one with the wolf has a nice contrast, and you even did a great shadow on the ground, but the body is bright and undefined by the location.
Keep up the good work!