Image formation is a very natural trait in human psychology. In our conscious state all our feelings, ideas and impulses manifest images. The genesis of an image is a cognitive imagination influenced by perception of an object.
In The Philosophy and Significance of Idol Worship, a Divine Life Society publication, Sri Swami Sivananda says:
“Idol is a support for the neophyte. It is a prop of his spiritual childhood. A form or image is necessary for worship in the beginning. It is an external symbol of God for worship. It is a reminder of God. The material image calls up the mental idea. Steadiness of mind is obtained by image worship.
The worshiper will have to associate the ideas of infinity, omnipotence, omniscience, purity, perfection, freedom, holiness, truth, omnipresence. It is not possible for all to fix the mind on the Absolute or the Infinite. A concrete form is necessary for the vast majority for practicing concentration. To behold God everywhere and to practice the presence of God is not possible for the ordinary man. Idol worship is the easiest form of worship for the modern man.
“A symbol is absolutely indispensable for fixing the mind. The mind wants a prop to lean upon. It cannot have a conception of the Absolute in the initial stages. Without the help of some external aid, in the initial stages, the mind cannot be centralised. In the beginning, concentration or meditation is not possible without a symbol.
Pratima (idol) is a substitute or symbol. The image in a temple, though it is made of stone, wood or metal, is precious for a devotee as it bears the mark of his Lord, as it stands for something which he holds holy and eternal. A flag is only a small piece of painted cloth, but it stands for a soldier for something that he holds very dear. He is prepared to give up his life in defending his flag. Similarly the image is very dear to a devotee. It speaks to him in its own language of devotion. Just as the flag arouses martial valour in the soldier, so also the image arouses devotion in the devotee. The Lord is superimposed on the image and the image generates divine thoughts in the worshipper”.
Excerpts from Hinduism Beyond Rituals, Customs And Traditions