Hi @mtiet!
Some themes let you add a special header image to the top of each page or post. Sometimes it might use the post’s featured image (if available), other times it might have a special system of it’s own.
Most event views, however, are best understood as a type of post archive – much like a regular WordPress tag or category archive – and so if the theme only exposes a means of setting the banner image for individual posts that won’t be available to you here and you’d need to do some customization work to achieve your goals.
Does that shed any light at all? If not, or if I’m misunderstanding the problem, could you help out by providing some more details and perhaps even a link to your site that illustrates the situation?
Thanks!