Visit https://www.openstreetmap.org/node/1937910403 to see an example of a speed camera (they are not shown by the standard OpenStreetMap layer itself, but still exist in the database). Marble's Vector OSM map theme should show them though on tile zoom level 17 onwards.
The goal of this task is to extend the Marble source code to extract the information of speed cameras and render them like other placemarks. Compare earlier commits like this one where OSM features were introduced to get an idea which places of the source code you need to edit (hint: not too many changes are needed, but they are spread across a couple of files).

The icon for the speed camera is available in the marble git repo at data/svg/osmcarto/svg/individual/speedcamera.svg