User-selected thumbnail for dock per VM

I have several Ubuntu and Windows VMs often open at once. The live thumbnail is basically a bunch of terminal windows. If I choose to display "OS Icon", they are all the same (per OS type), which also isn't helpful.
I'd like to be able to provide an image per VM to use as a dock thumbnail to more quickly identify the VM in the dock.
As a workaround, I've resorted to a live thumbnail, but change the background colors of the VMs, but that doesn't work well with an IDE maximized.

 

Hi Paul,
Please follow these instructions:
https://download.parallels.com/desktop/v15/docs/en_US/Parallels Desktop User's Guide/39647.htm

 

Yes! Thank you so much... I'll also take this time to re-read the manual; last time I read it was when I first got it v4!