|Right click on the image file and select Rename.|
In the box type the new name but make sure to include the image file type's extension such as:
mypic.jpg and make sure you name it anothername.jpg
If your PC is set to not show the extension then from the Explorer window go to Tools, Folder Options, View and remove the check/tick next to Hide extensions for known file types, Click Ok.
Next time you open the folder it will show the file's extension.
On the ones you lost you may need simply to rename them returning the file type's extension in order to be able to view them again.