e s c a p e reality

This isn't the erase the background it's where you have a image with no background like; and I want to paste it on a background like; but I can't figure out how to do it. On paint the normal regular paint there is a button the looks like this; that makes the image able to be posted on a different image/background. But I can't figure out how to do that on paint.net ~Danielle Please help my crisis.