Why can't I rename/modify photos/pictures on my PC?

Say I save a picture I find online and give it a title, like zom. Now in my image files, if I click to rename the picture to something else, a box appears saying that modifying this may make it unusable. Sure enough, if I do, it deletes the picture. I used to be able to change the title without a problem before, so why is this happening now? Trying to clean up my pictures, but this isn’t letting me. Help? Working with Windows XP.