How can I add a banner image in the confirmation email for a particular event?
If you want it in addition to the default/generic banner you can probably add it in at the top of the "Thank you and receipting" section of the event.

If you want to use it in place of the generic banner, you may need to customize the template with some If/Then logic, checking the event ID in the smarty template somehow. Here's an example for a Contribution page, perhaps you can do something similar in the Event page. (To find the variables you have to work with, try inserting {debug} into the template and then registering for the event to receive the email with the Smarty variables listed)

EDIT: Default message template can be changed like this

{if $event.id == '36'}
&lt;p&gt;&lt;img alt="" src="https://your.image.url.in.full" style="width: 600px; height: 300px;" /&gt;&lt;/p&gt;
{/if}