The simplest ways is to either upload them to a cloud account, or connect the phone to a computer, via USB cord and copy all the files into an empty folder on the computer.
Either way will save your images somewhere other than on the phone. You should also consider doing that with music, videos, contacts, etc. And, to be honest, unless the photos (and often videos) are used regularly on the phone, you should remove them and only keep them in the cloud account or on the computer. This is much safer with more 'personal' things. Don't need a lost or stolen phone having things on it you wouldn't want shared.