Visit the developer dashboard and click on the Edit link.
Scroll down (below the screenshots section), and upload the images for your extension:
The images will be put in the review queue after uploading it. If the images meet the criteria for promotional images, it will show up in the Web store within a few days.
You won't get any notifications when the images get (dis)approved. Send a message to cws-assets@google.com to query the status of your images. And no, you can't see the status at the dashboard, contrary to what the official documentation states.
