Hey, your stuff looks nice. I mean nice. Even the tearing zombie one is kind of nice, not that jaggedy terror-inducing thing:)
I don't think i can offer any tested solutions for the problem you mentioned with the last piece but i would experiment making the background more detailed/sharp edged and then play with the blur options.
In my opinion, the biggest difference between the background and your character at the moment are the edges. The colors, values (except maybe the pure white on the right) and level of details is good, it helps differentiate bg from the girl.
I'd want to keep bg on a seperate layer and try different degrees of blur. You can control how much you want to separate it from the Cassie.
One day, i will start with the faces and manage to draw people, like you and not some unintentional orc-human hibrids;)
Oh, and welcome to the forums!