Its config for windows.
Its set at the moment to show the file extension. I prefer this view, but you have to add the extension on the end when you rename files.
If you would prefer windows not to show this, then in a folder click Tools -> Folder Options, View Tab, then at the bottom of the current list view should be Hide File Extension of known types.
If this is ticked, Windows will hide the extension (jpg, bmp, png) and you can just type a name.
Also, if you are on XP you can highlight a load of images, press F2, type a name for them, and they will all be renamed to that name with a number after them - Nick, Nick(1), Nick(2) etc.
Hope this helps :)