White Outline On Render

how do i get rid of the white border that surrounds my render when i import it into my background?

 

I just magnify it alot and go around the edges with the erase tool. it is very arduous and time consuming but it gets the job done.

 

Or to prevent that problem, when you are creating your render, add a new layer of black behind the picture. This way, you can cut out the boarders while you are rendering it instead of re-rendering after finding out about it later