Well, I would not allow users to upload files, if they aren't registered users and logged in. If this is the case, create a subfolder for each user with the same username which has to be unique. Then a combination of a timestamp and md5 would do it. Combine the timestamp and the md5 with an underscore between. So it's easy to chop off the timestamp to compare for the image already being present. At least, that's how I would do it.