OK... here's the thing. Retake your picture. I know and completely understand where you are coming from - I do the same thing. I take a picture and spend more time trying to fix it than trying to take a good picture in the first place. This book I'm reading now says that the actual taking the picture is what you spend the least time on! What you spend the most time on is preparing and visualizing the shot, and that's what you need to do. To answer your question... If you're really set on changing the background I would definitely try to make sure the lighting matches the parts of the shot you want to keep, and make sure the shot you use as the background is your OWN. Other than that, just keep at it, and post your results!