I have changed mine so I'll try to explain the procedure. Someone may want to make my explanation clearer.
Open Settings. There should be an icon on the top or bottom of the Whisker menu for it or go to Settings in menu.
Open Sessions and Startup
Select tab for Splash.
Select Simple
(I think you have already done this if you are getting the Xfce mouse before your desktop.)
Click on Configure
Check Custom Image
Then you will have the option to find an image in your files. Click on the image you want to use.
Close Configure Simple window
You can then click Test to see if it fits in the area provided
Hope this works for you.
I am using
with the edges squared out in black using Gimp.
