I'm not sure how you are starting your graphic but one idea would be to start off with a black background as a default, then put your photograph in as a layer on top of it. As you work with the photograph, there won't be white pixels because you have that black layer behind it, which you can always delete or merge. Many people work on a background that is close to the color that they are going to set the graphic on so that the blending of the graphic with the background is more seamless. Does that make any sense? I could, of course, be wrong in my approach. I tend to be simple minded when it comes to this stuff.