Switchblade, good call on the first source! This was fun to dig up. Here are the photos. They appeared in a few books. There are a few other conceptual designs as well, most are in between the Japanese-style seen here and the final version, but closer to the final version.
First, from The Empire Strikes Back Sketchbook (Johnston, Joe and Nilo Rodis-Jamero. First Edition. New York: Ballatine Books, 1980).
These three are from the concept art for the Imperial walker.
Correct, the Cobra HISS-style walker seen here in the ESB Sketchbook is also featured in the old West End Games Star Wars Roleplaying Game. This same drawing is in "Elite Armored Unit" section of the WEG Star Wars Imperial Sourcebook (Slavicsek, Bill [as Development & Editing]. First Printing. Honesdale, PA. West End Games, 1989.)
This is an early concept for the Imperial scout walker:
These next three are concepts of the snowtrooper costume, as the following photo describes.
That finishes off the material from the ESB Sketchbook.
Now here is some more Johnston and Rodis-Jamero art from the The Art of Star Wars Episode V The Empire Strikes Back (Formerly titled The Art of the Empire Strikes Back) (Bulluck, Vic and Valerie Hoffman, First Revised Edition. New York. Ballatine Books, 1997)