You can download the vbulletin software from the members area and simply overwrite the images from the images folder from the download and back on to your server via ftp.
I know how to add a style but I can't find and don't have a vb orginal default style to upload and add, looking for afresh slate.
Using my previous method will take the default skin for your version, and reinstall it. Its a little white link under the style manager [Add New Style]
as long as the .xml file wasn't overwritten (doubtful). Just give it a shot and see if thats what you wanted.