There's nothing you can, or should, do about this. You application is no longer in the foreground, and the foreground app (photos) is asking for more memory. To supply this, iOS will terminate any apps running in the background. It doesn't matter how much memory your app is using, iOS will terminate background apps as long as the foreground app requires more memory.
Once your app enters the background you can expect it to be terminated at any time. To improve the user experience, you can look at Enabling State Preservation and Restoration in Your App. This in theory allows your app to start where it left off when the user comes back to it.