You can't export a mosaic image for display on your Helix unit, unfortunately. You can export in a few other formats - e.g. Google Earth, image files etc - and it is also possible to create images for display on Lowrance units - but not Humminbird.
To some extent I question the usefulness of using the full mosaic image on a unit, as with the limited resolution and screen size the map can end up looking cluttered. It can be more useful to either waypoint areas of interest or draw polygons around structures as these will be much more easy to read when you are on the water. This is possible on a Humminbird unit.