I mod a ROM Hack which doesnt have a cover so I just used the title screen which includes the games logo:

you could also do some file extraction shenanigans and make your own cover (I did this for example on https://www.speedrun.com/Akane_the_Kunoichi and pretty much the whole Monster Hunter Series)

"Pingus" doesn't have box art, because it's an open source downloadable game. So, as previously mentioned, title screen makes sense, or just the general game logo (even if it doesn't appear on said box) or whatever graphical art you think most appropriate.