Images padlocked in Desktop theme

Hi all, I'm a lover of desktop images, and overjoyed with Windows 7 slideshow. I routinely point it to a folder full of images, but recently discovered for some reason that a number of the images suddenly have a padlock against them. I notice this is showing in Windows Explorer too, but on checking the properties of affected images, there's nothing to show why i.e. they're not Read Only, Hidden or have any other unexpected attributes, and the security settings are appropriate to allow access by Administrators etc.

No idea what provoked this, although I've a vague suspicion it may have occured at a time when I was (vainly!) trying to get my Win8 laptop to connect to the home network, but never succeeded so unsure whether that could have had any effect?

Anyone come across this before or have any suggestions on how to fix?

Awesome - thanks SO much for your patience in replying :). I had reviewed the security options but couldn't see anything really wrong, however, there was a new Admin entry listed that I just deleted as it wouldn't let me change it much, & that seemed to sort things out. As I said, minor issue but nice to know where it was stemming from. Thanks again.