Fastest way is on the Preferences button, click the arrow to the right of it and choose "Get plugins to enhance calibre". Or use Preferences -> Plugins -> Get new plugins (same thing).
Choose "Search the Internet" from the list of plugins, click Install button. Make sure you select all the toolbars/context menus that you want the menu option to appear on and confirm all the dailogs. Close and restart calibre, and the plugin should now be available on your toolbar/menu wherever you placed it.
More information on the plugin itself can be found in the Plugins subforum on the first post of the
Search the Internet thread. You will want to use the "Configure plugin..." menu option to setup which websites you want to have appear on your right-click menu, you can rearrange them however you want or add your own, see the screenshots.
Specifically for google images I included a default search setup within the plugin for images larger than 400x300. However if you either add a new entry or replace that one with the url below then that will give you 510x680 for the title/author of the currently selected book...
Code:
http://www.google.com/images?as_q={author}+%22{title}%22&tbs=isch:1,isz:ex,iszw:510,iszh:680,islt:qsvga,imgo:1&safe=off
Be sure to check out the other plugins available in the plugins forum - see the
Index of plugins sticky page for the descriptions and links to the threads etc. Have fun, and if you have any questions about a specific plugin just post on its thread.