What do you suppose would be the problem if your images are not showing up in Google Image Search? Let's review the scenario: a webmaster has a site that has purely G-rated images, but his images are not appearing in the search results. He supposes that this could be because other sites, which don't use friendly images or text, are hotlinking to his images.
How do you test the theory? You could see which images do rank well and have another website start hotlinking to them.
You can also move all images to another directory and let Google re-spider the images.
But if you don't want hotlinking at all, just edit your .htaccess to prevent it.
Numerous other methods of hotlink prevention and discussion continue at WebmasterWorld.