The action buttons for "Suspend" and "Standby" don't have icons.
Created attachment 4541 Showing generic icon for Suspend in Action Buttons
added attachment as this affects me as well.
One has to use an icon theme that has system-suspend and system-hibernate. Unfortunately, currently neither gnome nor Tango have them. Faenza (and themes that inherit it) and elementary-xfce ( https://github.com/shimmerproject/elementary-xfce ) are themes that include those icons.
Just a heads up that one doesn't need to change the icon theme used. Just supply the missing icons. This can be done by symlinking xfsm-suspend to system-suspend and xfsm-hibernate to system hibernate in (icon path)/hicolor/48x48/apps/ or using other icons of choice.
Icons missing from the theme in use is not a problem that Xfce can solve.
Simplest fix seems to be to use xfsm icons, xfsm-reboot is already used so it makes no difference to dependencies etc. Patch is in bug #11513
As far as I see, neither system-suspend nor system-hibernate are part of the Freedesktop icon theme specification [1]. I agree with Chris that it would make sense to use the xfsm icons which are already there. [1] http://standards.freedesktop.org/icon-naming-spec/icon-naming-spec-latest.html