You can backup to iTunes and then restore the backup but it will include all data on your phone, not just photos. http://support.apple.com/kb/HT1766
Even if you plan to do it that way, I would also import the photos to a computer just in case something goes wrong. Instructions: http://support.apple.com/kb/HT4083
If you don't want to do the complete backup/restore another option is to import them to a computer as described above and then use iTunes to sync them back to the phone. The photos will then be in the Photos App but will not be in the Camera Roll. iTunes photo sync info:http://support.apple.com/kb/HT4236