Hello,

You can easily do that with Fatpaint.

1. Press on your silhuette object and in the Action menu, select “Create Selection”.

2. Then move the selection on top of the background you want. Then press the selection and in the Selection menu, choose “Extract selection”. Your select will now contain the background.

3. Then just convert the selection to a new object. Press on the selection and in in the Selection  menu choose: “Insert as a new object and deselect”.

That's it.