The easiest way would be to publish your site to a local folder. See the second method on this page...
Then open the folders for the individual pages to find your images and drag them out.
Otherwise you can open the iWeb Domain.sites package by control clicking it. The default location for this is Home Folder/Library/Application Support/iWeb
Make a copy of your Domain.sites file that is located in your Users/Home/Library/Application Support/iWeb folder and open it with the Finder, i.e. Control-click on the domain file and select Show Package Contents from the contextual menu.
The original image files that you used in the website will be located somewhere in that package.