After 1.9.7 -> 1.9.8 update, lost the ability to theme a panel icon.
In the past, it used to be possible to change an icon of the xfce4-screenshoter panel plugin.
There is a nice screenshooter icon in the standard HighContrast icon theme (attached to this issue). It's an excellent 256x256 icon with transparency. It used to be that it was picked up as a panel icon if a HighContrast icon theme is selected in the control panel.
However, this is not the case at the moment. Please, consider restoring the previous behaviour.