For those of you encountering "Error 403" when uploading a completed Roxio Photoshow even though you have uploaded other completed photoshows successfully. Several people have pointed out that the problem can be due to the use of mp3 music for which you do not own the rights. However, no one has noted that you also cannot use photos in your photoshow for which you don't own the rights.
For example, if you include even one jpeg image in your photoshow that you downloaded from the Internet (even if it is claimed to be available for unrestricted use) this will probably prevent successful upload. Try deleting any such downloaded images and your upload will probably be successful.