I'm impressed at the relatively ambitious illustrations you're attempting at your skill level. With or without the problems, that in itself is commendable. Nice work.
But there are a lot of problems. (Fig. 1)
• Ridiculous anatomy
• Incomprehensible perspective
• Terrible composition
• Blah blah blah, it's not that important to point out each and every little thing, like a visiting aunt pointing out all the areas you missed while sweeping.
I'm going to take a guess here and say that you do very little to no "construction" in your illustrations. Lack of planning can and will continually fuck you over, especially for full-scale illustrations, where the end result can look ridiculous if you don't take the time to carefully and gradually build your way to the end result.
Construction is important for helping you visualize and realize what it is you want to draw. The classic "I can see it in my head but not on the page" deal.
Construction varies wildly from artist to artist but it involves a lot of double-checking and guessing, whatever way you personally are comfortable with.
Surefire way to check that your illustration is advancing well and dandy is to use references (Fig. 2) - simply using photos of real-life and relevant objects and people to make sure your illustration adheres to a more naturalistic look - it doesn't have to be exact, of course (muscle, skeletons and fat form in hundred fucking billion ways, so trying to be "perfect" is never going to be achievable) but it doesn't need to be exact to be helpful. For example, the lizard character you drew has the torso of a bodybuilder but twigs for arms.
You could either have built up the rest of the character's musculature (the quick drawing under the bodybuilder, Fig. 2) or simply shrunk his torso to result in something more natural (the edit next to it, Fig. 2).
Also, flip your canvas often (Fig. 3). This is pretty miscellaneous advice, but if you don't then your eye adjusts to your drawing and perceives it as perfectly okay, even when it isn't. It usually isn't. Don't ask me how this works, it just does.
Onto some more theoretical stuff.
Your foreground (characters) and background elements are completely and utterly divorced from each other, and that makes for some pretty lame pictures. When it comes to combining the two, it's better to build from the ground up (again) and have them interact - going from a character portrait with unnecessary background detail to telling a small story, perhaps.
Basically, despite all the time and effort you've put into your pictures, you're not saying anything with them. They're just... there.
(Fig. 4) So you plan some more. Come up with ways to breath life into your pictures. Take inspiration from other illustrations, comic books, movie shots - stuff you want to portray in your illustration. A slice of life, however action-packed and fictional that life may be.
I've presented a small, quick, and example illustration incorporating those concepts. (Fig. 5) Just to illustrate the point. Makes for a nice cap to the critique, too.
Good luck man, and as before you're already taking some impressive leaps for someone so green. Keep at it, no matter what you do! Let your drill pierce the heavens and all that. I believe in you, dude.
Misc. Links
lovingthedark's basic art tips
A metric fuck-tonne of tutorials and links
Chris Beatrice's art blog. Already linked in the critique proper, may as well do so again. Swell guy.
Art101, a compilation of advice and tutorials by NG's finest.