The Unique Headers WordPress Plugin adds the ability to use unique custom headers on individual pages, posts or categories. You will see sections for changing your custom header when editing posts or pages.
If you would like to change the header image for a specific category or tag page, then you will need to install the Taxonomy Metadata plugin. WordPress does not support the taxonomy meta data which this plugin needs to use to store the custom header image URL. As soon as you install that plugin, the category image functionality should beign working.
The creation of this plugin was sponsored by the Norwegian Government Administration Services (G.A.S.) and Metronet. Coding assistance was provided from Jakob Schröter.
An experimental plugin for use with single posts when using this plugin is also available for testing.