Ah. The picture and the background actually don't go all that well, in my eyes. They most certainly could, but the linestyles and stuff are too different. The background is sharp and gritty - the girl is smooth and clean.
I'd say using some sharper, sketchier lines on the girl to match up with the background would help. Maybe you could even do some quick hatching or cross-hatching to shade, rather than using the soft shading. Both would bring the piece together better, so it doesn't look quite so much like you drew the girl then just found a background you liked and stuck it behind her.
And while I'm on shading. x3 You don't really seem to have any particular light source.. It all just kinda looks like a few grey chunks thrown in to try adding depth. Makes things pretty confusing to the eye when you try looking at her.