Where do I enter the game details banner image?

I'm trying to add my 1st adventure to the marketplace, but I keep getting the below message, which appears to be an error message.  I can't find anyplace which is titled as the "Game Details banner image." Game Details banner image must have an aspect ratio of roughly 15:7 (e.g. 750x350); similar aspect ratios like 2:1, 16:9, and 21:9 are also acceptable

Same issue here. Any solutions found yet ?

Hi King James and Rich B.,  If you go to Roll20.net, then click the name of your game you should see a spot for the image above the name of your game.  You can drag/drop from your computer or click Choose a File. Note: Due to a quirk in roll20's settings, different locations on Roll20 will display your game's image differently. Unfortunately there is nothing to be done about that except to decide which place you want to display it 'accurately'. The other places may squish it.