As far as I know, the only way to hide a picture is to put a dot in front of the file name, so picture.jpg becomes .picture.jpg. That's a hidden file in Linux, so it won't be seen by any app not looking for hidden files. (Putting a file named .nomedia [it doesn't matter what's in the file, it can even have 0 length] in a folder prevents files in it from being scanned, but that hides an entire folder and all its sub-folders.)
What Gallery shows (there's no folder for Gallery) is any picture files the Media Scanner finds. But you can't see hidden files on the computer (I just tried), so you'll have to find some way to cast the phone's screen or install a VNC (not VPN) server on the phone (and run it from the PC), or install
MyPhoneExplorer, use the PC 'screen' to see the phone and the mouse to move around and press things. (Once you're in there, you can go to Keyboards and check the MyPhoneExplorer keyboard, so you can use the PC keyboard to inpput text (copy, cut and paste work too).
Since there's no specific place the files could be, you'll have to look in every single folder you can get to. (It's going to take a long time unless you find them in one of the first folders you look in. Software that does a 'find' like that - starting at the root of the tree [yes, that's how computer science refers to a storage medium with folders] and looking in every branch, and every branch on that branch, etc., etc., runs VERY slowly. And that's a fast [compared to a human being] computer, keeping records of where it's been, so when it backs out it knows when it gets to the branch it has to look for more branches on.) So I hope you don't have to look too far.