Not sure if I understand you correctly.
(tray = panel)??
If you are asking about changing the icon on an app you put in your xfce panel. That can be done.
Its all about finding the proper icon file that you want to change.
In my case I wanted to change the 'synapse" icon (colorful and BUSY)
that showed up in my xfce panel to the 'synapse symbolic" type so it would match my other icons.
After a little 'playing' I was sucessful.
(click to enlarge)
The icon you want to change is in the 24 folder.
I renamed the icon I wanted to change to xxx.old
Placed the new icon in the folder and renamed IT to the name of the original icon.
In my case, I renamed 'synapse.png' to 'synapse.png.old' then added 'synapse-symbolic.png' to the folder, and renamed it 'synapse.png'
If your talking about changing icon in the menu's. I beleive its the same process, but a smaller icon (22 or 16? you have to experiment)