Background?

Hi,

I need help. I've got this picture. Almost finished. Only need to add the background. And that's the problem. Because, as almost always, I don't know what to do for the background. And therefore, I'm turning to you, my readers. If you have any idea, please, tell me, I'll appreciate it. 

Here's the picture:

Thanks in advance for any ideas you might give me. 

See ya.