Why does Paul call Jesus the “image of God”? 2 Cor 4:4
... the god of this world has blinded the minds of the unbelieving so that they might not see the light of the gospel of the glory of Christ, who is the image of God.
Let's note other verses that show Paul's understanding here. The words 'seeing' are used with little regard to sight of the eyes, but to know with the mind, to understand and apprehend who God is.
I've emboldened words to show a theme here.
Acts 26:18 to open their eyes, so that they may turn from darkness to light and from the power of Satan to God, that they may receive forgiveness of sins and an inheritance among those sanctified by faith in Me.'
Colossians 1:15 The Son is the image of the invisible God, the firstborn over all creation.
Hebrews 1:3 The Son is the radiance of God's glory and the representation of His nature
2 Corinthians 4:6 For God, who commanded the light to shine out of darkness, hath shined in our hearts, to give the light of the knowledge of the glory of God in the face of Jesus Christ.
While many references are made to sight and light etc - it is referring to knowing and grasping with the heart and mind what and who God is, and what He is accomplishing amongst mankind. 'Seeing' with the eyes has little to do with anything - esp. with regard to 'seeing' God. The 'glory of God' is a far grander reality than what one might 'see', but when the mind is opened too - physical sight of God's works is just a beginning to see the wonder of God and come to know who He is from the wonder of His creation.
Jesus showed miracles and works to help them, through seeing, they might then believe who he was and who he represented.
Believe Me that I am in the Father and the Father is in Me; otherwise believe because of the works themselves John 14:11
In fact, Is 53:2 reminds us, He had no beauty or majesty to attract us to him, so what we saw in Jesus is little to so with the eyes, but everything to do with the nature he displayed on behalf of his Father and God.
We see this further in the OP passage, 'the god of this world has blinded the minds' Again with the sight metaphor, but the message is about more important matters.
Jesus as the 'image of God', is a way to realise the true nature of God - shown in His human son. A son without sin or any corruption of the world but heavenly in nature - even though Jesus had his own will which differed to the Father's - yet was always brought into subjection.
Coming out of the OT structure of laws and death and endless sacrifice, there is much work to do to draw the people into a new way of light, love, grace, forgiveness. This is made (increasingly) obvious in Jesus who came speaking of the new Kingdom, the new life of spirit, being born from above etc. Again, speaking with sight metaphors, saying,
"If you had known Me, you would have known My Father also; from now on you know Him, and have seen Him." John 14:7
John 6:46 not that anyone has seen the Father except the One who is from God; only He has seen the Father.
No one has ever seen God. God's only Son, the one who is closest to the Father's heart, has made him known. John 1:18
When we see who Jesus is, what he did, how he did it, and come to know him, we know the Father also. As the reality of the New Covenant is being established in these new believers, they are needing to be retrained to know who their God is and what He is like. Jesus was sent to reveal the Father in ways never before understood with deep knowing Jesus makes possible by his example.
Luke 10:22 All things have been entrusted to Me by My Father. No one knows who the Son is except the Father, and no one knows who the Father is except the Son, and those to whom the Son chooses to reveal Him."
Paul draws them into the new reality of who they now are - and we who follow.
Such confidence we have through Christ toward God. Not that we are adequate in ourselves to consider anything as coming from ourselves, but our adequacy is from God, who also made us adequate as servants of a new covenant, not of the letter but of the Spirit; for the letter kills, but the Spirit gives life. 2 Cor 3:4-6
Now the Lord is the Spirit, and where the Spirit of the Lord is, there is liberty. 18 But we all, with unveiled face, beholding as in a mirror the glory of the Lord, are being transformed into the same image from glory to glory, just as from the Lord, the Spirit. v17
Man, made in the image of God (Genesis) occurs only in (not by) Christ. Without Jesus - 'in the image of God', we cannot fulfil our destiny to be in God's image.
Just as we have borne the image of the earthy, we will also bear the image of the heavenly. 1 Cor 15:49
Romans 8:29 For those God foreknew, He also predestined to be conformed to the image of His Son, so that He would be the firstborn among many brothers.
This was the plan all along - God as Creator has not stopped creating, We are His creation, brought to completion in Christ - who is the image of his God.
We, as the 'many brethren', are included in Jesus - the image of God, to also be image bearers as planned from the start,
Further with regard to 'seeing' with the eyes. We remember Israel and their exodus from Egypt - they saw amazing things that we'd think would change them forever. No, days after the miraculous crossing of the Red See, they were grumbling again. They saw - but they did not know or understand - they were blinded by the veil God had allowed to remain.
But their minds were hardened; for until this very day at the reading of the old covenant the same veil remains unlifted, because it is removed in Christ 2 Cor 2:14, Col 1:26, Eph 3:3-6, Rom 16:25-27