ergophobe - 6:32 am on Dec 22, 2010 (gmt 0)
The thing is, won't Google image search index those images based on the fact that they appear on the page, without ever crawling the image directory? Hmm... maybe not, since I guess it wouldn't be able to see the image and wouldn't follow the src link.
On the other hand, if you watermark your images with your URL, maybe you'd like to get them hotlinked. It's worked wonders for the Church of the Great Spaghetti Monster (it's an explicit strategy of theirs to encourage hotlinking).