iCloud storage backup
I'm not exactly sure about your specific scenario yet, so here's some information about a few possibilities:


If your photos are stored in iCloud Photo Library, then they're not also stored in your iCloud backup.
If your photos are just in your camera roll and you don't use iCloud Photo Library, the photos/videos would be contained in the iCloud backup (also true for an iTunes backup). In this case I'm not sure that deleting your iCloud backup will help, since the new backup would just want to back up all of the photos again :)
If you use iCloud Photo Library, deleting the photos will delete them permanently and from all devices, so doing something like this is likely not what you want to do!
If the photos aren't on your device now, and they're only in that backup, then you definitely don't want to delete that backup. (I don't think that's the case here, but I want to make sure I say it!)


Be sure to tread carefully before deleting things and make sure you know whether you use iCloud photo library before figuring out what to do! (Also I should note that iCloud Photo Library is different than Photo Stream and photo stream is NOT a permanent backup and shouldn't be used as a backup.)