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That is presumably a feature of the app you were using to view the pictures, since it's something I've never seen in almost 11 years of using Android.
So if you can tell us which app you were using it will be easier for someone to help. Even if it's one which came with your phone do not assume it's the same as on anyone else's phone: manufacturers load their own apps, so tell us the name of the app and, if it's something generic like "Gallery" that you didn't install yourself, tell us which phone you have.
If you'll forgive me for being pedantic, the built-in Gallery is a specific app. And the Gallery is one of the apps that's most likely to vary between devices, since most manufacturers have their own version.There is no specific app I am using, just the built in Gallery.