( I am not native speaker and not good at English, so maybe there are some mistake in my sentence. sorry)

Some days ago, I request a new game. It was submitted and I became moderater. Now I edit game rules and themes. And I am trying to edit main image of page. (The image displayed under the game title. Left upper side in the page.) The image of big "?" image is now displayed

At first, I thought "Icon" is that, but it changed only page tab's icon. Of course, banner, logo, background are not too.

How can I edit it ?