If I do a right-click > Properties on the Xfce4‑menu‑plugin icon, and click "Button icon", preview is only available for the already selected icon (xfce4_xicon1.png by default).
Jannis, Nick: I'm just using exo's preview widget here -- any ideas?
Xfdesktop uses garcon to generate the menu in 4.10+ so this issue should be resolved.