There are approximately one zillion apps that want to back up your pictures to the cloud, but Amazon's solution was just updated with a new timeline view mode, which is actually kind of cool. So what's it do? The timeline makes it easier to jump to photos from months, or even years ago.
It's not clear at first if the timeline is some specialized area of the app, but it's actually just built into the main photo view. "Timeline" refers to the slide-out bar on the right that lists all the months and years that appear in your photos' EXIF data. Swipe that tiny drawer open and tap on a month to zoom there. You can further refine the location by long-pressing on the month to get individual days.
The app does automatic uploads in the background with all the usual settings to limit uploads to WiFi networks. Amazon offers 10GB of free storage for photos, which isn't a ton if you're uploading 16 or 20MP pictures. Still, storage is cheap if you want more.