Thank you for you nice question. A general answer i can give for now but i can also include some resourses.
just as we cannot see the soul (spirit) leave the body when someone dies we cannot see God's spiritual body with mundane eyes , so He is very kind to appear in a fmaterial orm that is according to scripture. Not an idol, an idea that is made into God but a form that is made after the authorized description of God.

www.krishna.com/main.php
Krishna's Deity Form
Krishna’s form is spiritual; it is not made of matter. In our present condition, with our material eyes, we cannot see Krishna. So He descends in an apparently material form as the deity in the temple to allow us to see Him and serve Him directly.

For God there is no difference between material and spiritual energies, since they both come from Him and are under His control. He can use them as He sees fit. By Krishna’s will, the apparently material form of the deity is actually spiritual.

In the Vedas, Krishna reveals how the deity is to be made and worshiped. When Krishna’s form is created in this authorized way, He is personally present to accept service from His devotees.

Krishna states in Bhagavad-gita (9.26), “If one offers Me with love and devotion a leaf, a flower, fruit or water, I will accept it.”




and here is a tribe totally dedicated to this activity of deity worship please post the same question there.
deityworship.tribe.net