How to stop Google Image indexing (images only)

From what I have read, a robots.txt file may not be sufficient, and it certainly will not be sufficient right away. It will not produce any major results at least until the next time your site is crawled by Google.

In addition to the robots.txt file, updating your .htaccess file (if possible) may provide a solution. I don't have a lot of knowledge on the use of the .htaccess file, but to forbid hotlinking of your images uses the following:

Or, if you want to just prevent google.com from hotlinking, try something like this:

I've had all my images protected for years. I'm not 100% sure exactly how Hot Link protection works but I think the search engines are indexing images and saving compressed copies on their servers, not hot linking to the files.

If you don't mind me asking, why do you want to stop Google from indexing your images when you're getting traffic from it? Image traffic isn't usually high quality stuff, but it's better than nothing.

Like others here, I am unsure exactly as to what your motives might be. However, once captured by Google, it may be some time before they come back and take note of your robots.txt file, although Google does respect your wishes here. (Unlike some search engines, I hear.)

To frustrate visitors who use Google's captured links, you can always move your images to a different folder. Better check all your links, though! Mscreashuns has the right idea for limiting access - you may like to read a bit more about the technique, and how you can adapt it to your needs, on this article on Smarter Image Hotlinking Prevention at A List Apart.