Quick look of the pdx is sometimes showing the launch card, but not always. So you shouldn't rely on it to check if your launch card work on not.
To check the launch card, the best way is to put the pdx in PlaydateSDK/Disk/Games and to launch the simulator. Your game should be listed in the system menu.
I never tested imagePath with just an image but I would think you will have to drop the file extension (the png file will be converted in a different file format when the project is build)