The complete background for a custom dashboard is not shown when accessing the dashboard as the NoLoginUser. The amount of the background displayed seems to be dependent on the length of the longest item on the dashboard. When I minimize the longest chart less of the background image is displayed.
WORKAROUND: Use the "fixed" layout when creating the dashboard and add a text entry to the bottom of the dashboard.
You can add a text item to the dashboard by dragging the "Drag into portal to add text..." entry under the "Text" section of the General tab onto the dashboard. You can show just the text and not the title of the text item by editing it and un-checking the "Show title" box (see the add_text.pdf attachment).