keyplyr - 9:46 pm on Nov 18, 2012 (gmt 0)
If they are exclusively mobile networks, it's most likely an app that is using the image. I have thousands of musician photos. Media players are always scraping and/or hot-linking to these images for their album cover art.
I check for referrer, and if the request comes from a source other than my own domain, I switch the image to an image file with a warning message.
I also use a mod_rewrite condition that blocks several UAs from getting image files (but allow them to play my media.)
BTW - IMO you'd be blocking a large percentage of human users with the code in your post. They will view your site with NO images. You have to allow your own site (all versions) as referrer.