@alix2020,
You think it would be hard to find image files that you had "hidden" in a Windows folder, but the truth is that using either the command line or else Windows Search a person can find every image file on a disk, in seconds. Your friend probably did that as soon as he got the disk in his computer.
You should never assume that other people know as little as you do about computers. In fact, that thought leads me to this: if you have ever used peer-to-peer filesharing software like Limewire, and you shared your whole C drive, (many people do!) that means
all your files are visible, and anybody in the world can just search for picture files, Word documents, your credit card details, etc etc.