When it comes to Buddha Image, there are two things we need to know. First, the non personal symbol and second, highly developed representational art with an abundance of figures.
We have to understand that for a long period of time or during Hinayana doctrine the Buddha image was the non personal symbol. How do we present in visual form something entirely transcending human vision without dragging it down to human world. This form had to be visually impressive, yet spiritualized to an extreme.
He who like the sun has gone to rest is comparable to nothing whatsoever. All ideas are nothing, as bearing upon him. Hence, all modes of speech are, with respect to him, unavailing. This is the way the Sutra Nipata, a text of the Pali Canon, describes the Buddha after his death or Nirvana.
The modes of speech unavailing applies equally to representation to art. For this reason, the Hinayana represented the Buddha as a symbols alone and not in person.
It was a continuation of the old Brahmanic practice of not representing deities as persons but of worshipping them as best in the form of symbols. Some of the symbols used were:
a. The Tree of Enlightenment
b. The Wheel of Doctrine
c. The Throne
d. The Pillar encircled by flames
e. The Footprints
f. The Stupa
Many of these symbols relates to the important events in his life. For example: his enlightenment is his division to take up preaching and his entry into nirvana.
However, these gradually follows a shift in the center of gravity from the symbol to the image, a portrayal of a person. At Gandhara, they are depicted in human form.
When Mahayana Buddhism was established these images of Buddhas and all ideas and concepts were used as a pathway, something temporary and non real, to help an individual to attain enlightenment, to become a Buddha, by meditation, rituals because Mahayana believes that every individual has the potential to be a Buddha.
It is necessary to understand this important paradox before one can understand form and nature of the Buddha represented as human being. These are some concepts of Mahayana that were of fundamental significance in directly the development of the Buddha image.