I’ve used http://hugin.sourceforge.net/ to do things like this, in combination with gimp.
Hugin basically can be used to account for moving the camera during the shot, and to output a multiple layer image where all the layers match up perfectly. You can then use the layers in gimp to make the rider images be on top.
If you use a tripod or hold the camera nice and steady, you can often do it easily enough in gimp alone (free thing that does quite a lot of what photoshop does), I just had some where I’d done some pretty major panning/tilting during the shot.