While the site states it's a Zoological Society member benifit - it appears to be available to anyone visting the site (I've been a member for 35 years)
The WebMaster listens to the membership - over the past year they have moved the "minimal" amount of "advertising" to areas that are generally Icon free on most desktops.
http://www.sandiegozoo.org/membership/members_wallpaper.html

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