The Concept of Deity Worship
Devotees of Krishna worship Him in various Deity forms with the understanding that God is the source of all energies—the Absolute Truth—and can appear before us in any form He likes. Everything about the Supreme Person, including His form, is spiritual and worshipable.
Vedic scriptures first of all define who is Supreme and who isn't. They also describe the science of Deity worship in detail—what Deities should look like, how they're supposed to be made, how to worship Them, and the benefits of such worship.
Krishna, or God, is pure spirit. He's not made of anything material. But in the material world, we can't see spirit. All we can see is matter. By agreeing to appear in the form of a Deity, Krishna allows us to see, honor, and serve him, even while we're still in material existence.
(Image depicts Krishna devotees chanting in kirtan before the temple Deity.)