The only option you have here is to mark your blog as not wanting to be indexed by the search engines. You do this Dashboard -> Options -> Privacy. This will create Meta tags for your site that should prevent most offical spiders from indexing.

The issue though is that this will also remove your blog from teh search engines completely, not just the images.

Since this is a shared environment, being able to modify the robots.txt file would mean you would be modifying it for everybody here and they would be doing the same to you.

Hope this helps,
-drmike

"To remove an image from Google's image index, add a robots.txt file to the root of the server. (If you can't put it in the server root, you can put it at directory level.)

Example: If you want Google to exclude the dogs.jpg image that appears on your site at http://www.yoursite.com/images/dogs.jpg, create a page at http://www.yoursite.com/robots.txt and add the following text:

User-agent: Googlebot-Image
Disallow: /images/dogs.jpg

To remove all the images on your site from our index, place the following robots.txt file in your server root:

User-agent: Googlebot-Image<
Disallow: /