Nope, uploads are not versioned. *Articles* in the Image: namespace *are* versioned. In other words, the text you write on an Image: page is versioned, but not the uploaded file.
Images sit in the /wiki/images folder. When you upload a new version, the old file is overwritten.
DanB
-----Original Message----- Does MediaWiki do that itself? There are file histories on all the [[Image:X]] pages, aren't those what you want?