So this sounds like a stupid thing to write, since this seems like a no brainer.

BUT - I can't get my cover art to show. I upload the file (512 x 512 px PNG (8-bit or 24-bit) or JPG) and the thumbnail works, but I can't get the picture to show up as a cover photo.

I can print it with a script and it works fine, but what am I missing? is there an option somewhere that needs to be enabled?
I have gone through all the options I could think of related to this issue under "game", but no luck. other computers in my class have the same problem. So what am I missing?

I am using the web version of quest on a mac.

So just to be clear, you have published your game already? Thwe image will only be there for a published game, and I not sure if you can update it through the game once it is already published.

You can add it via the website site instead: Go to the game listing (the page with the big "play" button). Click on "Edit picture", then browse to the image, click Okay, then Submit. This should be the same regardless of editor or computer; it is uploading a file to a web site.