Edit your game in launchbox, find "Additional Apps" and add WincdEmu there with the correct parameters, probably something like C:\Program Files (x86)\WinCDEmu\batchmnt, with the path to your image file as the application command line parameter. Check the "automatically run before main application", checkbox.
You can add another app that launches after your game closes, which unmounts the image again, using the checkbox "automatically run after main application". I think that would be something like C:\Program Files (x86)\WinCDEmu\batchmnt, with /unmountall as the application command-line parameter.