From Google Webmaster Central's "Prevent your images appearing in Google search results":
To remove all the images on your site
from our index, place the following
robots.txt file in your server root:
If you wanted to block specific image types from Google, you could use this:
Other search engines don't offer such control, but most will will honour robots.txt files, so blocking directories of images (i.e. the gallery URL and the directory storing the images) is the way to go.
You may also wish to disable hotlinking of images to prevent people linking to your image files directly from other websites, which could cause them to be picked up by Google et al.
If you've done all that and you're still seeing your images in Google's results, you might like to check that the robots.txt file is in your root directory, and that it's publicly accessible in your browser.