I like the portraits, but I think they'd benefit from a stronger directional light-source.
It's easier to explain this in pictures then in words, so here is a rough paint-over:

Don't be afraid to stick the darkest and lightest spots next to each other either, especially with metallic objects.
Hope I didn't step on anyone's toes by posting this, I actually think the portraits are great: solid anatomy and lots of character and the lizard dudes are pure awesome. The lighting could just use a little love

.