One thing that was not working with my Gentoo installation was the information for image dimensions (and image data in general) in Dolphin. The resolution is to rebuild Dolphin with the semantic-desktop use flag turned on.
echo "kde-apps/dolphin semantic-desktop" >> /etc/portage/package.use/dolphin
emerge --ask kde-apps/dolphin
I thing that this flag should be on by default when building a system with KDE desktop profile or at least mentioned somewhere in the Gentoo wiki.