I have my Win-10 setup to login without a password, and the login screen displays a circle with some goofy thing in it. So I figured I'd see if I could replace it with something a bit more festive. So off to Control Panel/Account Settings/ and there was an option to change the picture from the stock MS one to any image I wanted. I picked one and sure enough the stock one got replaced. But there was no way to crop or position the picture. Worse, if the picture was square, it didn't fit very well in the forced round hole. If I cropped the picture before I selected it to fit in the circle it looked funky, so I just figured I'd live with the stock image, and proceeded to delete all the images I had selected. Everything went fine, until I went to delete the current image. I thought I'd be able to delete it and it would return to the default image, but it didn't work that way. No matter what I did, I couldn't delete the single image in the circle. OK, I'll play your silly game, and created my own circle with a 50% tint and replaced the last one that I couldn't delete. Sure enough my 50% gray circle replaced it, and I could finally delete the image. But when I re-booted the 50% gray circle I created was too small, and for some reason it was a bit off center. Having spent way too much time with something that should take no time, I just gave up, and now have an off center 50% gray dot staring at me when I log on to Windows 10 to remind me how lame MS Windows can be sometimes. A little searching turned up the pictures are stored here: The default user account picture images are in the hidden system folder below. %ProgramData%\Microsoft\Default Account Pictures Your Current user Account picture is stored in the hidden folder below. C:\Users\Public\Public Account Pictures\S-1-5-21.......(GUID for user account) Even though you can see the Current Account picture you cannot delete them. You cannot change the properties of the directory where they are stored either. AFAIK there is no way to go back to the stock account image that comes with Win-10 and there is no way to delete the last single image. You must have at least 2 images to be able to delete one of them. It's what I call the Hotel California setting.... You can check out anytime you want, but you can never leave.
You can edit your picture on your Microsoft Account page. You just log in and go to the Your Info section. There you change your picture.