Welcome to Android Central! You probably have to delete the auto-backup folder in your Google+ Photos in the cloud. Go to your Google+ page on your computer browser and navigate to your Photos. Find the Auto Backup folder and delete it. If the photos persist in your phone's Gallery, then go to Settings>Apps>All, select Gallery, and Clear Cache/Clear Data, then Force Stop. When you open the Gallery again, the photos will repopulate, this time without the Auto Backup folder.