I've always like the one at the Opeth Nectar mailing list in Egroups.com under the files section. If you're not registered, it's worth it for this one. I tried attaching it, but it's too big.
Here's the web address:
http://groups.yahoo.com/group/opeth-nectar/files/opethwallpaper.jpg
I believe you need to sign in/register though before you can go to the page.
Anyone has/ knows a link for Blackwater Park wallpapers ? I think it's the best metal artwork ever, and would love to have all those pictures as wallpapers. Thanks.