I found this file in package x11-themes/gentoo-artwork-0.4.1: /usr/share/pixmaps/gentoo/1280x1024/gentoo-freedom-1280x1024,jpg I Think there is a comma... "," instead of a ".". The right name might be: /usr/share/pixmaps/gentoo/1280x1024/gentoo-freedom-1280x1024.jpg ;-)
Fixed in CVS, thanks for reporting this :)