Thumbnail Rollover

Hi,

Rather than have to click on an thunbnail in a post(I think this is already available tho I have not tried it) to see the full size image, I would like it to be a rollover.

The CSS can be added easily enough, but I don't have a way to make it easy for members to add the html to their blog post.

Here is an example:
http://50mpg.org/blog/?page_id=43

Kind Regards,
Larry