Here is a brief audio on the second commandment, the title is a bit misleading as that is the last bit but it has a good general argument why pictures of Jesus are forbidden; does not get into mental images; but if you don't believe the first the second is immaterial.
http://www.sermonaudio.com/saplayer/playpopup.asp?SID=12416191271
Here is Ralph Erskine's comment along the topic.
Our Lord charges them with mental adultery, that look on a woman to first after her (Matt. 5:28). This sin may be as really committed, though a woman be not present to be looked at with the bodily eye. If a man shall frame an imaginary idea of a woman in his mind to lust after her, it is mental adultery. Even so it is mental idolatry to form a picture of Christ's human nature in our mind, by an imaginary idea of it; and so to make that the object of faith or worship. To form that picture of His humanity in the mind, is a mental looking to it: And to make that the object of faith and worship is a falling down to that image; and committing mental idolatry with it. Indeed I know not who can justify themselves, and say they are free of this sin in some measure. It is too natural, and, I believe to every saint, as long as he is in the flesh, and hath a body of sin and death carrying about with him. But I think it is possible for true believers, to take up a vast difference between that fancy or imagination of Christ as man, which can lead a person no farther, and that faith that apprehends Him as God-man, and sees the glory of God in His person. The former is nothing but a shadow, and a mishapen apprehension, a notion of something in the head; and yet put in the room of Christ. But here, to anticipate what will further occur: “Can you think of God-man,” says Mr. Robe, “without thinking on Him as man? Is it not necessary in the nature of the thing, that you have an imaginary idea or conception of His being man; otherwise you cannot conceive of His being God-man.” Ans. I think, neither the Godhead nor the manhood of Christ can be rightly conceived, but by faith. It is strange to allege, as if by one means we conceive believingly of His manhood, and by another of His Godhead; as if the one were by an imaginary idea, and the other by faith and spiritual illumination; or as if the imaginary idea, which is a natural act, was helpful to spiritual actings of the soul, when it is rather the quite reverse. The spiritual believing view of Christ as God-man, through the illumination of the Spirit, is the only means for enabling the natural faculties to any right thought or conception of Christ, and without which even His humanity is quite imperceptible, as it is the object of faith. This is one of the things of Christ, which flesh and blood cannot, but our heavenly Father can reveal, and which the Spirit of Christ only can show unto us (John 16:14). The natural man receiveth them not.
The sight of what is corporal, as the object of the imaginary idea is, can never in itself fit and prepare us for seeing what is spiritual; but rather darkens the understanding, and makes it unfit; even as the god of this world does thereby blind the minds of them that believe not. The image of Christ’s natural body in the fancy darkens the view of Christ, as the image of God, by faith. These two images cannot stand together, no more than Dagon and the ark. Dagon must fall, if the ark come into the heart.
Pectora nostra duas non admittentia curas.
Again, let a man have an imaginary idea of Christ’s human nature, now exalted to heaven, and sitting at the right hand of God, and on the throne of God; he forms the idea of a man, and the idea of a throne on which he sits. I would ask, whether the idea he hath of a man, be any better than the idea he hath of a throne; or if any of these ideas give the least help or assistance to his faith; or rather, if they do not cloud his mind, and contribute to make him have a gross, carnal, and unworthy notion of Christ? Can he in that glass see anything of the invisible glory of God in Christ, as the image of the invisible God?
Peter Martyr, loc. com. p. 155, speaking of images of Christ, says, “If the bodily presence of Christ was a hindrance to the apostles, and the sight of His human nature an impediment, unfitting them from receiving the Spirit, till once He went away in that respect from them (John 16:7), how much more will images of Christ prove impediments.[”]
We have no other glass to see Christ in, but the gospel; no other eye to see Him by, but faith. If the eye of sense and imagination come between, there is no seeing of Christ by faith, till that eye of sense be shut.
Again, to conceive of Christ as man, is carnal worship and idolatry, when this imaginary idea of Him as man is brought in, as helpful and necessary to faith or worship. Which two I mention together, because faith is a special leading part of divine worship....
Ralph Erskine, Faith no fancy: or, A treatise of mental images, discovering the vain philosophy and vile divinity of a late pamphlet intitled, Mr. Robe's fourth letter to Mr. Fisher: and shewing that an imaginary idea of Christ as man, (when supposed to belong to saving faith, whether in its act or object), imports nothing but ignorance, atheism, idolatry, great falsehood, and gross delusion (Philadelphia, 1805), 64–65.
