The fist thing you need to do is to create some Header background image. It should be in some 1920 x 300 pixels or in similar size for the best results.
Then you need to navigate to WordPress admin -> Media Library and upload your header image inside. Once the upload is complete you should see the image and its URL. Copy that URL for later use.
Next step is adding some additional CSS, so navigate to Theme Options -> General -> Additional CSS and copy/paste this code and save changes.
background: #FFFFFF url("replace_with_your_image_url") no-repeat fixed top center !important;
Please note that you need to alter the text “replace_with_your_image_url” with the full image URL path that you have copied earlier and save change.