Use Entities.add_image
: http://www.sketchup.com/intl/en/developer/docs/ourdoc/entities.php#add_image
The second argument dictates where it should be positioned. If you need to rotate it, transform it afterwards.
model = Sketchup.active_model
entities = model.active_entities
point = Geom::Point3d.new(10, 20, 30)
image = entities.add_image("Shapes.jpg", point, 300)