I have no idea about just what Dr. Ambedkar may have had in mind in regard to Asoka,
but I do think that the question of the emergence of images and idols in India should be empirically focused.

See, for instance, Michael Willis,
The Archeology of Hindu Ritual: Temples and the Establishment of the Gods (Cambridge 2009).

Although Willis prudently avoids the question of beginnings, some interesting suggestions may be gleaned
from section 2.7 "From Private Sacrifice to Public Spectacle," pp. 113-122.

One may also wish to consult, for evidence of deity-worship in early Buddhism, Robert DeCaroli,
Haunting the Buddha: Indian Popular Religion and the Formation of Buddhism (OUP 2004).

Matthew