Did the Buddha discourage antropomorphic representations of himself? The first anthropomorphic representations of the Buddha appeared centuries after his death (see this related question). Buddhist art then was "aniconic", insofar as the Buddha was only represented through symbols such as the Buddha's footprint or the Dharma wheel. In the Wikipedia article on Greco-Buddhism, there is a hint as to why this might be so:


  [The] reluctance towards anthropomorphic representations of the Buddha, and the sophisticated development of aniconic symbols to avoid it (even in narrative scenes where other human figures would appear), seem to be connected to one of the Buddha’s sayings, reported in the Digha Nikaya, that discouraged representations of himself after the extinction of his body.


This assertion isn't, however, accompanied by a reference to the relevant portion of the Digha Nikaya, and it is contradicted in Donald Lopez's latest book, From Stone to Flesh: A Short History of the Buddha (University of Chicago Press, 2013): 


  [N]o proscriptions against making images of the Buddha have been located in what scholars would regard as an early Buddhist text, nor have any prescriptions for his representation been found.


Is there actual textual basis for the claim that the Buddha discouraged antropomorphic representations of himself (in the Digha Nikaya or other early Buddhist texts)?
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.