Welcome to Android Central! Do you mean that the pictures have backed up to your Google account, so you want to delete them from local phone storage? Open your Google Photos app, tap Menu, then tap the Free Up Space option. This will detect which photos are backed up to Google Photos, then automatically delete them from the phone. Remember that you'll still see them in the Google Photos app (and the Gallery app, if it's synced with Google Photos/Picasa), but those will just be cloud pics, and not taking up any significant space on the phone.