In a normal search on Google Images, you get a list of thumbnails that point to a frame page: you'll see another small version of the image and the webpage that included the image. To see the full-sized image, you click again. You can get the full size image directly with a single click.
To achieve this for Firefox, you can install Customize Google extension that, among other features, rewrites links to point straight to the images. If you already have GreaseMonkey installed, you can add the script from Patcavit Website listed below.
For Opera, save the script from UserJS.org to a folder on your computer and make sure you set the option from [Tools/Preferences/ Advanced/Content/ JavaScript options/My JavaScript files] to that folder.
Internet Explorer too has a GreaseMonkey clone, called Trixie. It supports most GreaseMonkey scripts, so you can use the script for Firefox in Internet Explorer too.
If you wish to download the entire search results at a time, GoogleGrab is just for you. It downloads images obtained using Google Image Search and even supports batch search.