How do I Insert a custom header image?

I am using the theme “Clean Simple White” (it’s a basic theme similar to thesis/thematic etc).

I want to know how to insert a custom header image instead of just having the title across the top.

What code do I need to insert – and into which page (stylesheet.css? or header.php? or what?) I am familiar with basic html but not css/php, so I know how to upload images and then point to them using html – but how on earth to insert it into a template? Flummoxed!

The site in question is https://www.macnessa.com