In the Sri Dakshinamurti Stotram, the Lord is presented as the source of spiritual knowledge. The vigraha itself is visualized and presented that way.
The ashtamurti is a traditional form of Ishvara. It includes the five elements which, according to the Vedic model, constitute the whole universe. Besides the elements, we have the sun and moon. The moon represents all the satellites and the sun, all the luminaries including planets and stars. These make up seven elements of the ashtamurti and the eighth is you, the jiva who is looking at the Lord. It cannot be better presented. This Lord who is all, who is the maker and the material cause, is seen in the form of Dakshinamurti.