Hello araenel,
Thank you for liking and using our Reef theme. You can do it by adding custom CSS from appearance > customize > additional CSS section.
Add your CSS just like this:
.wrap-inner-banner {
background-image: url(YOUR-IMAGE-PATH.PNG) !important;
}
OR by customizing the theme. If you are planning to customize the theme, we always recommend to create a child theme and customize it.
Please check the following reference link to create the child theme.
https://codex.wordpress.org/Child_Themes
Hope it will help. Let us know if you are getting more problem.
Thanks,
Keon Themes Support Team