Since you asked for a critique, I'll try my best with it. I'm going to do it under little sub-headings in order to make it a little more organised. Anyway, here we go.
~ WHAT YOU DID RIGHT ~
Well first of all, this is a very striking piece. The lines constructing the junkyard that pull towards a vanishing point direct your eyes straight into the centre of the piece. The positioning of the characters is well balanced without looking too stock and generic (as some film posters can do) and the designs themselves are nice. There's almost a Burton-esque feel to the two human characters and it fits well with the dark setting. I also really like the design of the robot, particularly his face, plus his pose is dynamic and tells you a lot about his character. As for the big creatures in the background, they have very unique designs and are intimidating yet weirdly funky at the same time.
Lastly, it's good that you drew some detail into the junkyard without over-doing it, plus the deep shading in the path helps draw your eyes towards the characters.
~ WHAT COULD BE IMPROVED ~
I think the biggest issue here is anatomy. There's a saying that goes 'You need to know the rules before you break them,' basically meaning that you need to study anatomy before finding a way to stylize it. Again, you have a very nice style, but knowing anatomy could add to it. This is particularly a problem with the middle giant creature. The foreshortening of the arm and hand looks good, but the hand itself looks a little off, especially the middle finger. Also, the male human character seems to have two left hands. I'm going to post a link to an anatomy course that I took recently that might help you.
This piece could also use more shading to make it look more striking. To further bring the eye towards the characters, I suggest blacking out the sky and making the distant junk yard heaps quite dark. It may also be worth shading the junk surrounding the central characters a little and only leave a small ring of white around the all on the floor. That way, the male character's torch light will be more noticeable if you leave the beam white and anything around it shaded lightly. In any piece you make, it's always important to draw attention away from unimportant information towards what you want people to see.
~ LAST MINUTE NITPICKS ~
This is just to cover any last things I may have noticed. As well as anatomy, it may be worth looking into how the folds in clothes work and skirt/dress styles. You've done well with perspective, though the milk carton looks a little too raised to me.
~ IN CONCLUSION ~
Overall, well done. I think this is a good piece with good composition and the tweaks that need making are only fairly minor. Don't feel discouraged by any criticism as it's there just as a suggestion to help you. Please let me know if you're going to put an updated piece here on NG at a later date :)