How do I get my own image into the NTS KBento template header.

I've been trying to get back into a Kunena forum for a while now and just bought the KBento template for it. From the documentation, I learned how to insert the category images. That works just like it used to with my old NTS template. But how do I get the header image shown in the template "with umbrella and KBento text" into my template? Of course, I also want to use my own image here. Unfortunately, the documentation only lets me define the background color of the forum header. It would be great if someone could help me out here. The recommended image size in pixels would also be helpful. Thanks in advance.