A solution to the original question (Change Header Image on all posts in a given category) is as follows.
Install the plugin Category Featured Images – this gives one ONE Featured Image for each post in a given category. Then go into Weaver Xtreme and look for the section at the bottom of your post entitled Single Page View: Featured Image and select from the drop down Header Image Replacement. You will have to do this for each post in the category – there is not a Bulk Edit option to do this. But it is not a major issue if you plan on this from the beginning of the “post thread”. You can now using the Category Featured Images change all Header Images for a given category.
If you want a given post to have a Featured Image and add that image to the post it will display BUT the Category Featured Image will no longer be your Header Image – it will return to the default Header Image. You could always use a media insert to put in a “featured image” as a substitute in your post.
BTW do not use the plugin Featured Images for Categories quite different plugin!
Hope this may help someone – Ken