I am running Cantata, an MPD client, which sends notifications of the currently playing track along with the album cover image. Greyscale images don't display, however; they generate the error "gdk_pixbuf_new_from_data: assertion 'bits_per_sample == 8' failed".
gdk_pixbuf_new_from_data documentation says that "currently only RGB images with 8 bits per sample are supported.". Maybe we can just ignore the image if bits_per_sample is different than 8.
Sorry, guess this isn't an issue specifically with xfce4-notifyd.