If someone is interested I used this cool widget:
[Custom widget: Dashboard Link] - thx
changed the code to:
<button class="dashboard-link" ng-click="sendCmd(config.habpanel_item, config.dashboard_name); $root.settings.background_image = config.bg_img">
and added a new setting with id bg_img for image address (e.g. /static/bg.jpg)
Now Im able to change the dashboards with this link and the the background also changes.
Lets start with my HABpanel floorplan! 