Note: This functionality is available in Parallels Desktop for Mac Pro Edition only.
If you have two or more virtual machines with the same guest operating system, their Dock icons look the same, and you may get confused. To change the virtual machine icon, do the following:
Copy a new icon you want to use to the clipboard (select the icon and press Command-C).
Right-click the virtual machine which icon you want to change (virtual machines are usually stored in the ~/Parallels folder) and click Get Info.
Click the virtual machine icon in the top left corner and paste the new icon from the clipboard by pressing Command-V. The icon will change in Finder and in the Dock.
To return the default icon, repeat step 2, click the virtual machine icon and press Backspace.