It might be possible to change the header pic on the Main "index" page, and the Dashboard, but the other pages carry the same css. Only the Main, and Dashboard pages are defined in css. See below. These are the direct css paths. Note that the first two are defined as the Dashboard,and Index page. The third one is the standard css from Oxwall for all other pages. Since the first two are defined you may be able to add a different header image for those two pages. All others will have the same header.

html body.ow.venus_plus.base_user_dashboard div.ow_page_container div.ow_canvas div.ow_page.ow_bg_color.clearfix div.ow_header div.ow_header_pic

html body.ow.venus_plus.base_index_page div.ow_page_container div.ow_canvas div.ow_page.ow_bg_color.clearfix div.ow_header div.ow_header_pic

html body.ow.venus_plus div.ow_page_container div.ow_canvas div.ow_page.ow_bg_color.clearfix div.ow_header div.ow_header_pic