Of course, the obvious piece, but have to mention it, is that you can add and change the graphic for an event by simply creating an event and click on upload a new graphic or link to an existing graphic under the Attach a Graphic: section.

Now, after stating the obvious, you will now see that if you select No Graphic, your event will display the following graphic:

But say you don't like that graphic and you don't want to keep clicking upload new graphic (or link to the existing graphic) when you add an event.  In that case, if you go under Platform Settings (only available to admins) you will see a section under Events where you can upload a new default graphic.

Once added (keep in mind you can only attach a PNG), you will now see this as the default graphic for all events you add where you don't upload or link to a graphic.