Back up your photos without taking up storage
Bear with me while I explain the difference between the iCloud Photo Library and My Photo Stream. iCloud Photo Library uploads and stores your photo/video library to your iCloud account when you back up your phone (which counts against your 5GB of free storage). But My Photo Stream automatically uploads photos and sends them to all your devices when they’re connected to W-iFi, and doesn’t eat up any of your storage.
Think of it like this: the iCloud Photo Library is like paying to have all your photos stored in a safety deposit box, and My Photo Stream is like handing a secure copy of your photos to each of your dearest friends, and asking them to look after them for free.
Both are solid options, but one that doesn't cut into your free storage is always better. To set it up, go to Settings > iCloud > Photos then turn off iCloud Photo Library and turn on My Photo Stream (also, make sure to do the same on any other iPhones or iPads associated with your Apple ID).