It may not be...
It may not be your downloaded file. You can download again from this source and do a md5 sum check. If the download file is correct, have you burned it as an image file, at a low speed using a good CD/DVD disk?
By "invalid signature" do you mean that the disk boots but the media check function on the CD/DVD reports a fault in the burned media?
http://software.opensuse.org/