hmmm... i just noticed u wanted a critique? if you dont mind i do have a suggestion <img src="e.deviantart.net/emoticons/s/s…" width="15" height="15" alt="" title=" (Smile)"/>
when i was first learning about manipulations, my biggest struggle was trying to the make the separate images look like that belonged against the background, or when they happened to be layered over eachother.they never seemed to blend properly.
MY TRICK: Use the blur tool. zoom in on a object and use the blur tool along the edges. set it about about 30%-38% . Trace the edges more than once if u have to. For Example: follow along the edges of the aliens tentacles, keep in mind you might have to do this more than once.
i think it helps with my work, so I hope it benefits you <img src="e.deviantart.net/emoticons/s/s…" width="15" height="15" alt="" title=" (Smile)"/> good luck