If you want to prevent Google from indexing your website / blog images, you can use the noimageindex value in the meta section of the head portion of your HTML page:
<meta name="robots" content="noimageindex">
However, if other sites directly link to your images (hotlink), then Google will still index them, as they explain on their help entry on the subject. A more stable way than above to keep Google and others away from your images is to put all images into a special folder on your server, like "/images/" Then, you can add a robots.txt file to your root and put the following text in it:
User-agent: * Disallow: /images/
Conforming user agents will now not crawl that folder, and so they won't be able to generate thumbnails from your images to put on their server. In theory they would still be able to link to your file – e.g. if they found the link to it on another website – but in practice, they currently won't.
Source:→ Google Blogoscoped
Google, Indexing, Crawling, Google Bots, Image Indexing, Meta Tags