hi elvee, theere's some links about skinning:
for Mac:
http://www.resexcellence.com -> This is the most visited, and here you can find any Mac skinning related site, but i recommend:
http://www.iconfactory.com/ -> the really masters on 'iconism' on any platform
http://www.theapplecollection.com/ -> the most Mac-innovative and larger wallpaper collection around.
http://www.panic.com/index.html -> the creators of Audion and major OSX goodies.
for Linux:
http://www.kde-look.org/ -> for KDE desktop, where a skin is viewed by the most 'anxious' people.
http://art.gnome.org -> for gNome desktop. The most elegant skins and wallpapers for Linux. It is very new and needs to be 'refined', but it will be the standard gnome desktop customization website.
http://www.themedepot.org -> for gNome and (less) for KDE desktops. The most new, and intended to be the 'skniz.org' of Linux desktops.
I don't know about artists, but there's some interesting site's of Everaldo, Pixture Studios, Mormegil, Xymantix (only for say some of them).
Thanks