Open the existing image in your program and create new layers with the desired text and images on top of the original image (or just create a new image of the exact width/height at 72dpi) - 720px by 180px looks to be correct. Use a solid fill layer to hide the old image. Flatten image and save with the same file name.
If the image is not managed somewhere in WP and is coded into the theme - upload via FTP in place of that file or Upload via Media Manager and get the new link and place in the template file in place of the original image link. Refresh browser.