Those are probably cached thumbnails related to some app you have on the phone that are mistakenly showing up in the Gallery. If the directory where these images get saved don't have a file called ".nomedia" in it, then the system will automatically add the images to the gallery database. App developers are supposed to know to put a .nomedia file in the app's data directory, but they don't always remember or know to do that.
Select any of those thumbnails and look at the Info/Details. Which directory are they located in? That can give you an idea of which app they're associated with.