Tagging Images

I got really excited when I saw TagCow, a site which uses Mechanical Turk to tag images in the thousands, but was then disappointed when I realised that the tagging only applied to images on their site. What I really want is a similar service where I could upload thumbnails of my images to get tagged but then have the tags incorporated into those images' meta data on my own hard drive. I am pretty good at tagging but with 8,000 images and growing it is a daunting task!