How to change dock icon

Dear all,

Parallels desktop is a great software however i totally dislike the monitor icon with the two red lines in my dock.

I managed to customize my virtual machine icon and also the Parallels icon that appears permanently in the dock - even if the software is not running.
As soon as i start up my virtual machine the customized dock icon disapperars and the original icon shows up again.

Has somebody an idea which icon i need to change?

I have been through all *app packets and changend nearly every icon but it comes back again and again....

Thanks in advance!

Regards
Achim