For those using Windows 10 with Open Shell, you might encounter the ff. issue:
Install Transmission for Windows.
Add the icon from the menu to the task bar.
Install another app, like a third-party image viewer, and the task bar icon changes to that of the viewer.
In case this happens, you can do the ff.
Locate the Transmission app in the programs folder.
Right-click on the exe file and create a new shortcut. For the icon, select what's offered in the exe file itself.
Add that shortcut to the menu, and then add that menu item to the task bar.