SOund to me like you have file extentions turned on

Open a folder, pictures for example (any folder will do) and go to TOOLS at the top of the window. Select FOLDER OPTIONS and then the VIEW tab. Scroll down and you will find and entry for "HIDE EXTENTIONS FOR KNOWN FILETYPES" - make sure this is ticked (I am guessing it won't be ticked) and click ok.

Now you should be able to rename a picture file without having to enter the file extension.