General Electric has a large number of images and animations on their Ultrasound Site. In particular, they have a large image library, and the "Anatomy of an Image" which describes details on imaging parameters and processing techniques.

A fascinating description of using ultrasound for neurosurgery.

ATL Ultrasound has a large number of images in their reference section.

Dr. Woo has a site with many links, and the interesting history of ultrasound.