The FDO notification spec version as returned by GetServerInformation is 0.9. This should be incremented to 1.2. Xfce4-notifyd already supports the following hints which are spec version 1.2 and later: image-data image-path transient
Simon Steinbeiss referenced this bugreport in commit 0273941351c9aad1fd76a6fdaaba6960915a2961 Bump notification spec version to 1.2 (Bug #13578) https://git.xfce.org/apps/xfce4-notifyd/commit?id=0273941351c9aad1fd76a6fdaaba6960915a2961
Thanks for noticing, should be fixed now.