incrediBILL - 6:44 am on Mar 19, 2012 (gmt 0)
Whether the subdomain is virtual or not is meaningless, if you can upload a file, which you obviously do with images, you can upload a robots.txt file as well.
If you can't upload a robots.txt file, you certainly couldn't upload an .htaccess file that would make sense either, as both should go in the root docs folder.
You can either put both files on your server or neither, see what I'm saying?
In the event that I'm wrong, happens on occasion, this should go in the .htaccess to stop indexing:
<Files ~ "\.(png|jpe?g|gif)$">
Header set X-Robots-Tag "noindex"