[Theme: Weaver II] how can I add another header image?

You can likely do this by editing the header.php file or using the image as a background in the CSS. But you may want to post on the Weaver forums as they will be able to give you more specific help with this.

No php editing needed
If you mean you want to use an image In place of the Site Title and Site description, there are manu ways to do that in weaver.

You first Hide the Site title and description with the option found in Main Options > Header

Then you can insert your image at the top using one of the methods below.

Use the Header widget Area to insert your image. You can create more than one header area if you want to position that image in different ways (several thread on the subject on the weaver forum)
You can insert the html code for your image in the Advanced Options > HTML Insertion > Site Header Insert Code box
for more support come to the forum at
https://forum.weavertheme.com/