How did you do the rotoscoping? If you used the Roto Brush tool, it's a good idea to freeze it (with the Freeze button) to prevent oddities later.
Did you use masks?
Anyway, once you have someone cut out, simply put the new background on a layer beneath the person. I'm not sure what else you are having problems with.
Somehow the image i put as a backround shows up only there where i cut the person...
there are a few ways to rotoscope so it will be helpful to show us what exactly are you doing with screenshots. the way you are describing your problem in the last response seems to be related to how your mask mode is set. when you use masks and set it to Add (the default) - it will show only the image. when set to subtract - it will show everything else but the cut area.
it appears you are new to the this whole process. rotoscoping is one of the toughest challenges you could face when it comes to visual effects. you should read enough about it before you dive right it because 90% success ain't gonna cut it (pun intended). here is some training for you: Managing and animating shape paths and masks in After Effects , After Effects CC tutorial: Rotoscoping with paths | lynda.com - YouTube