HT4759: iCloud: System requirementsLearn about iCloud: System requirements
If you created an iCloud backup your camera roll photos should be contained in the backup. Restoring to the backup should recover them, but you will have to restore to the entire backup. If you have more recent data on your phone you will have to save this first, or create an iTunes backup to restore to later (by right-clicking on the name of your phone on the left side of iTunes and selecting Back Up).
When ready, go to Settings>General>Reset and tap Erase All Content and Settings. You will then go back through the setup screens again, and when given the option, choose Restore from iCloud Backup. When complete, if you made a current backup in iTunes, import the photos to your computer (see http://support.apple.com/kb/HT4083), then go back through the same process again, this time choosing Restore from iTunes Backup and restore your phone to it's current state. You would then need to put the photos on your computer in a folder and sync them to your phone by selecting the folder on the Photos tab of your itunes sync settings and syncing your phone.