Midrash Rabbah (Exodus 34):
When God gave Israel the Bible, they would not have survived His approaching them in His full might: "If we hear the voice of Hashem, our God, anymore we shall die." (Deuteronomy 5:22). Instead, He approached them according to what they could endure. The Bible says: "The voice of Hashem is power." (Tehillim 29:4). The verse does not say "His power," but "power." Hashem approaches each being according to the power it is capable of enduring.
The meaning of this is simple. The Bible takes into account man's limited ability to grasp abstract concepts. While we were commanded not to associate any picture with Hashem, we must absorb that command according to our own ability to understand. Man is limited by his five senses, with which he perceives the world. He is no more able to transcend those senses than to shed his own skin. When Hashem speaks to us, He employs our own language and concepts. He adapts the celestial concepts we must absorb to our intellectual faculties.