I would not use an app for this. If you have a mac you can download your pictures with "Image Capture" which comes with all macs. Put them into folders and store on your iCloud and also on at least one (pref two) external hard drives. Every few months download you newer photos and repeat the same process.
This will free up space on your phone and save your photos safely. If something happens to your phone, which is always a possibility, your photos are probably the one thing you will most regret losing.
To use image capture, just plug in your phone, open image capture and you will see your phone, click on it and you images will appear. Just drag click all and drag them into a folder. I am surprised at how many Mac owners don't use or know about this handy pre-installed tool.