HI, I created a banner/header image to the specified size (1920x120px) and I’ve added this to the site.
However when I look at a page with it looks like the header has been enlarged OR the breadcrumb trail/navigation section has been pushed up covering the lower part of the header image -checkout http://demo.katcentre.org.np/who-we-are/
Note that I’ve tested using the default supplied header image and they too get obscured or enlarged.
The theme will make the header area as tall as configured in the options. You are currently displaying a 250px tall image in an 120px tall area. You might want to adjust that value.