You could use an application like iBackupBot, to delete the photos, but then I will know if the backup to restore from it
If you decide to do previously make a copy of the backup, the app itself allows you to make a duplicate of it, but you can also do it manually, but do not specify what your operating system, or via time machine if you use a mac
If you do not recognize itunes, you could use itools not let you delete the photos, but the applications will indicate what occupies each, and you can even make a backup of them. Once removed you could do a backup in itunes
In any case always keep a duplicate of the backup that you have now
Greetings and welcome :)