The image you are referring to is set via your themes
style.css file and not via the WordPress admin or in your theme options. You can find the file here:
Here is the CSS that is used for this section (line #1305):
padding: 60px 0px;
The image itself (http://promisingcoins.com/wp-content/themes/structural/images/breadcrumb.png) is a little strange as it only has 2 thirds of the actual image containing the texture/effect so can't really be resized easily to fill the full width of the site using CSS.
In order to change the image, I would strongly recommend using a child theme or adding CSS via your theme options (if there is an option to add custom CSS) rather than editing your theme files directly, to add the following CSS code to your child themes style.css file or custom CSS in your theme options: