Err... I'd clear cache
If i were to guess, the images are saved locally in cache, with some "pointers" pointing toward the location of storage to retrieve them for display when needed. And in this case, the pointer remained the same while the file at the location at which they point to got changed and... this happened.
There is also a good likelihood of your computer being infected with some malware, because that USUALLY doesn't happen, as cached files are not supposed to have the same/repeating names etc, so that the system knows when a file is changed and no longer valid to be used by such pointers. This is, btw, how a lot of virus operate, replacing a healthy file with a similar "evil" one that does additional harm when they are executed in place of the healthy file.