Did the Buddha discourage antropomorphic representations of himself?
A direct quote from the Digha Nikaya that explicitly expresses a prohibition or even a discouragement concerning his antropomorphic representations has not been found yet, as far as I know. 

In Religions of the Silk Road (Munshiram Manoharlal Publishers, 1996), Yuvraj Krishan maintains that the aniconic period is due to a Vedic heritage: Vedism was aniconic because it considered the highest gods as ultimately transcendent and formless. His contention is that when Buddhism was in its infancy, those who were faithful to the growing tradition were careful to act within to the Vedic tradition to avoid any normative displacement. So it should not surprise us that the earliest representational art appears at the geographical fringes of the Indian subcontinent (Sri Lanka and Gandhara). The earliest source for this thesis that I know is John C. Huntington's 1977 paper titled "The origin of the Buddha image".

Although there seems to be no known explicit doctrinal injunctions against the representation of the Buddha in anthropomorphic form, there is an  oblique statement in the Vinaya of the Sarvastivadins (first called attention to by Arthur Waley) which may be interpreted as a prohibition and consists the only textual source I have found to this effect. In this passage, which deals with the decoration of monasteries, Anathapindika asks: 


  World-honoured one, if  images  of yours  are  not allowed  to be made, pray  may we not at least  make images  of Bodhisattvas in attendance  upon you? (apud Waley, 1931/2)


Although this statement addresses the theme of iconic representation of Buddha, it is an isolated case and cannot really be construed as a prohibition. Therefore, Huntington's theory that the aniconic representation of the Buddha in early Buddhism was a result of Vedic traditions and not specific doctrinal prohibitions seems to be the best explanation.



Further reading: 


Rob Linrothe, 'Inquiries into the origin of the Buddha image: a review', East and West 43, 1/4 (December 1993), pp. 241-256.