Why do old photos / email images keep getting downloaded into a gallery folder - HTC One M9?

[AC Question]

Every week or so I find random old photos or image attachments from ancient emails being being downloaded into a sub-folder called 'All Downloads' in the standard Gallery folder on my HTC One M9. These can range in number from about 15 to 150. I delete them, then a week or so later it happens again. How do I stop this from continually happening - what settings do I need to change and why is it dragging really old (from several years ago) photos into this folder, and where from? Any help in sorting this is very much appreciated.