You could make sure that all of your images are in 1 specific folder, for example yoursite.com/images/. Then add the following to your robots.txt file: User-agent: * Disallow: /images/
I think the following code will work for you User-agent: * Disallow: /images/ User-agent: Googlebot-Image Disallow: /
Go your Robots.txt file and block robots to index your image folder by User-agent: * Disallow: /images/
For Google Crawlers or Spiders where all the images you want to block in one folder i.e., images User-agent: * Disallow: /images/
The alt text is on the current html page being crawled. The image is in a folder where the robot must not go, so that image will not appear in the image index. The page text content contains the alt text - so it going to be part of whatever is indexed. The title on an image or link does not get indexed.