When I download my pictures from my Samsung Galaxy S3 onto my desktop, hundreds of screen shot pics download. These are those annoying pop ups that come up trying to get you to select the app to purchase. They are not in any picture folder in my phone - I only see them when I download all pictures. How can I find them in my phone and delete them?
Just a bit of warning: Even if you do delete these pictures that come from in-app advertisements, you'd have to block those apps from accessing the internet otherwise they'll download those pictures again. [Unfortunately, that's another question entirely]
Otherwise, to fully delete these pictures, it'd be best to use a third-party app, as follows: http://features.en.softonic.com/how-to-permanently-delete-files-on-android
The reason why I suggest using an app instead of, say, deleting them right off your SD card is that most of them won't be on the SD card. Most apps usually hide them inside their own personal data folders, so the link above should help you.