Hello,
You need upload image (for example, image.png) to wp-content/themes/woopress/images folder and add following code in custom.css file
.header{
background-image: url(images/image.png);
height: auto;
width: 100%;
}
Here is a video tutorial how to create custom.css: https://www.youtube.com/watch?v=Qok2zRedRMY&feature=youtu.be.
Regards,
Robert Hall.