I figured this would be worth bringing up. The Elder Council group on Facebook has a number of galleries for each game in the series, though the images mostly consist of cover scans and official screenshots. However, I did find something very interesting in the Arena section. A massive, stunningly rendered illustration of the Halls of the Colossus which was probably digitized and touched up to make the ingame background seen in the game. It was pretty common back then (especially with adventure games) for people to paint the backgrounds and other closeups, and it certainly produced much nicer results than traditional pixel art could. I've downloaded the high resolution image and dumped it here for anybody who's interested.
http://imgur.com/O4u7j.jpg
Very interesting! I didn't know any snippets of Arena's artwork survived. Does anybody know what medium (i.e. oil painting) this is, by the way?