Your rewrite can just take /? on it to make it optional.
The source image paths are irrelevant. Unless its somehow matching your rewrite rules which is very easy to do. Your current rules only check to see if a file matches, otherwise it goes ahead and rewrites. Assuming the style is overall the same, not matching a filepath for an image would attempt to rewrite it. I wouldn't think that would cause a 404 on a page though; all that should do is show a broken image link since the image itself is a 404.