1. Landscape:
It took me a couple of seconds to figure out that I was looking at hills. The sky is too dark to see the mountain clearly in the background, and the lighting on the hills is... not right.
2. Elephant:
Creepy. The way the right eye almost overlaps the ear on the large face doesn't look right, maybe because the eyes are proportionally smaller than on the tiny face. Also you could probably find a better colour to outline the tusks and eyes than black.
3. Cute little robot:
I really like this guy's design. The new look is much more metallic, but the old animation had more life to it, though the hat seems to be jumping around his head much faster than the the gun is firing.
4. Murder dog:
I love this thing. The lightest tone of brown isn't bright enough to show up much and the ears seem to be stuck to his body rather than his head. Also it might look better if the ears flapped up a little when he falls back down.
5. Pig:
It's way too dark to really see anything without zooming right in and putting up the brightness, and even then it's hard to tell what you were trying to do with this picture.
6. Computer guy:
This one doesn't look bad. The desk seems to be lit mostly from the top so you should make the top of the speakers, monitor etc. much brighter to match. Also his leg might be a little too long, and the chair looks like it's too low and too far to the right.