made visible, made known...

John chapter one seems to have some clues that may answer your question. Verse 18: "No one has ever seen God; the only God, who is at the Father's side, he has made him known." It seems to agree with other scriptures to think that the "seen" in that verse and the "image of the invisible" in the Colossians text mean something more than just visual imagery. John says "no one has ever seen God, but Jesus, he has made him known." It is almost as if He uses "seen" and "known" to mean the same thing. John's first epistle has some very similar ways of talking about seeing and knowing. In fact, He goes even farther to include hearing as a part of the revelation of God that is Jesus Christ. "That which we have seen and heard (this is a reference to Jesus), we proclaim also to you, so that you may have fellowship with us; and indeed, our fellowship is with the Father and with his Son Jesus Christ." It seems that what John is getting at and what we might suppose that Paul is getting at when he calls Jesus the "image of the invisible God", is this: Jesus is to us, in every way, the revelation of God, the image of the God who was previously invisible and is still invisible to those who "do not know him (John 1:10)." But not just visible with our eyes, especially our eyes, we've never laid eyes on Jesus after all, but with our hearts! Seeing God, according to these verses, means knowing God through Jesus. Knowing his character, his fellowship, his love, his fatherhood and perhaps most importantly, being like Him (Colossians 2:6). Hopefully it isn't too much of a summary or a stretch to interpret the Colossians verse this way: Jesus, his person and work, makes knowing God possible and effectual. He is the image of the invisible God.
Does this get at an answer to your question? Thoughts about those thoughts?