what make and model of iPhone do you have?
were the pics that you lost originally taken with that iPhone's camera?
Ok and how long ago did you make this back up?
Ok, what you'll need to do is to reset the iPhone and restore from that backup.
You should be able to restore from iCloud by restoring the device. It's important to note that this will erase the device so if there is anything on there which you currently wish to keep, let me know. Assuming the backup is still in iCloud, you should be able to do the following to return to the last backup:
then tap “Erase all content and settings”
When the device restarts, it will display the Setup Assistant. In the Setup Assistant, sign in to iCloud, tap “Restore from a Backup,” then choose from a list of your backups in iCloud.
Your device is restored as follows: Your settings and accounts are restored from the iCloud backup you chose. Your device restarts and begins downloading your purchased music, movies, TV shows, apps, books, Camera Roll, and other content.
You may be asked to enter passwords for iTunes, App Store, and iBookstore accounts to restore purchased items. If your device can’t download the version of an app that was backed up, it downloads the latest version.
Progress bars appear below Home screen icons for apps being downloaded. To make an app a priority for downloading, tap its icon.
To check if the information on your device is fully restored, go to Settings > iCloud > Backup.
If the photos were backed up properly then they should restore just as you had them.
Ok, it will take time for all your pics to download. It's best to leave it connected to wifi and it normally only takes a few hours at most.
No they should appear in the camera roll slowly as it proceeds. It may not begin downloading photos until all apps have finished downloading though as everything does not get downloaded at the same time.
Any pics that were not taken by the camera and found in the Camera roll, such as those synced from another device or computer, may need to be res-synced or downloaded.
It said problems backing up OR problems restoring from back up?
Ok, is the one pic the only thing you seem to be missing currently? ...and if so, are you low on storage on the iPhone?
Ok, that's probably why you are having trouble restoring the back up.
You're best bet at this point, if everything else looks ok, is to remove whatever you do not need to gain back some space and you should be ok moving forward.
Are you missing more than the old pic you mentioned previously?
iCloud will automatically delete older backups as it creates new ones. You shouldn't need to delete back ups unless it has trouble making new ones.
If everything looks good except for the one pic you mentioned, then I recommend going through and deleting any restored apps that you no longer use. If you sync photos to a computer you may wish to do that and delete older photos to save space but do not delete any you wish to keep unless you have them stored on a computer, etc...
If you no longer see the backup restoring new photos as time passes then that may be correct yes. What has happened as time passes is that your back up has become too large to successfully and completely restore. This also often happens when you are close to running out of storage and perform a software update. The newer update takes more space, leaving less room to restore. It's good idea to try to keep at least 500MB to 1GB of free space on your iPhone.