It's a mystery - but no-one can deny they make a great job of it.
If you want your images to show up, the most obvious thing would be to make your image titles very descriptive, and not worry about their length too much (I guess most of us use short image titles for convenience) but there's no reason why you couldn't name one: seaside-lighthouse-sunny-aardvark.gif etc. And put further hints in the <alt> tag
(all assuming its a picture of an aardvark at the seaside of course)
On the other side of the coin, I spent a lot of money on a graphic of a familiar item, that I knew would show up well in image searches, and really didn't want stolen. So I ensured its title was highly non-descriptive - e.g. image1.gif, with an alt tag saying just 'banner.' It won't stop individuals stealing it, but it will stop *thousands* of people stealing it through an image search under the assumption it is somehow in the public domain!