Dropbox's Carousel is a photo and video managing app where you can backup photos and videos. It also allows you to automatically remove an old photos that is cluttering up your phone. You need to download and install recent version of the app on your Android and iOS device. If you already have a Dropbox installed on your mobile, then sign in to your account or provide your credentials to access your account. It has a feature located in the "Settings" menu that allows you to free up the space. After a few moments' calculation, you'll be told how much space you can clear. Tap "Free up (x)MB od space" button to delete the local copies of the photos. Your deleted photos are still safe in the cloud and on other machines where you have connected your Dropbox account. You can view your deleted photos from Carousel, but your device's internal storage is now free.