You cannot open the existing skin files. You can only right click in the hangar (in-game) and choose "save default skin" or whatever has "save skin" in the words.
That is only the default though, not all the other user-submitted custom skins.
You will find that in a subdirectory of the \skins\directory.
Please see the sticky post in the custom sounds and skins forum that describes how to start skinning.