The margins do not have any effect. What's up there?
The margins do affect elements according to their positioning type. With relative positioning the element will be left docked to the so called DOM tree and can be displaced by the margins. Under certain circumstances this can affect surrounding HTML elements.
With absolute positioning the element will be undocked from the DOM tree and the margins are distances to the border of the browser window. With this positioning type you have to choose two reference edges from where your image/module is positioned. They must not be opposed! The two unneeded values you can simply set to zero for not being evaluated.