The resolution handling in Unity for web browsers seems to be un-intuitive. I've seen quite a few that do weird things. In this case, your game didn't show the full game until I switched to fullscreen. My screen is only 1366x768 though.
Also, did you get permission from Wizards of the Coast for the images? If not, I'd suggest you quickly replace them with look-alikes or similar royalty-free images. It's unlikely WotC will notice, but the rules for nearly all jams requires that you not use images you don't have the rights to use. It is a nice concept, too, so it'd be a shame if they did complain.