I'm running Xubuntu 12.04.1 with kernel Xen4.1-amd64 and the following problem is mine:

Nearly all icons have been replaced by the default "no icon available"-icon, even on the login screen and the all icons in software like e.g. Firefox and the icons I manually assigned to launchers, too. Changing the icon theme does not seem to affect this.

The only icons that did not disappear are: LibreOffice, gThumb, Simplescan, Ristretto and in the XFCE menu office, tools and graphics.

How could I bring the icons back?

EDIT: When I tried to use Ristretto the following error showed up:

Could not load image-loader module /usr/lib/x68_64-linux-gnu/gdk-pixbuf-2.0/2.12.0/loaders/libpixbufloader-png.so Unable to open shared-object-file I/O error

(Had to translate this to English.)