Google recently released a new Photos app, with free unlimited cloud storage for 1080p video files and photos up to 16 MP in quality. It makes it easy to browse and search through all of your photos, but you may have discovered some unfamiliar photos after installing it. This is because the app searches your entire device, and displays the photos it finds in the app. At first glance it may seem to be an error, but there's a way you can quickly find out where the photos came from.
In the Google Photos app, tap on the photo that you want to get information on, and then select "details" from the menu. This will tell you where the photo is on your device, and what folder its in. Then you can go and either delete the folder, take it out of your Google Photos collection, or just choose to not have it automatically back up.