The Convention Cover appears in the Event Portal as a banner for your event in the event list and on the event's information page. 

Search for your event in the event list. 

Then click on the three dots in the upper right corner.

Three icons will appear. Click on the "pen" icon.

The "Edit event" view will now open. 

The "Upload" button opens the dialog window of your browser. 

Select your desired graphic and upload it.

Now click "Publish" to save your changes.