I suspect two sources, the Internet leaves behind pictures when you visit different sites, pictures that were in advertisements and you didn't pay any attention to.  Also windows and other programs have pictures that you don't normally see unless you access the program, but they exist and when you look for jpg or bmp or png or gif and some others they show up.

A free program I like is https://www.voidtools.com/downloads/

It's the fastest program I've ever seen to find a file.  You can put *.jpg and see every file with that extension and then sort by size or date or name.  It's handy to get rid of pictures or junk files as well as find them.