Hi KAV, I use the same theme, and I think understand your problem...

U ned to go to Admin Panel, Appearance, Edith Theme, Theme Sethings, Page header image...

First of all, let me tell you, that the header of this particular them is composed of two images, one is that you want to change, that says: "Welcome to our Social Work Community", and the other is the image heather background you can change too, in Page promo image.

The first image is in a PNG format, important!!!, respect PNG format. And this image is in transparent way.

IMPORTANT!!!...

You can Upload other PNG image with a different height, try to respect the width, but if you Upload a PNG image with less, more less heigh, your can Upload the OTHER with the same heigh and get more space for the rest of your site.

The trick, is just to upload a transparent PNG image, with the height U choose, so the  "Welcome to our Social Work Community" image wont appear. And U can have a better impact design heather.

I hope be usefull... bye.