What does "whose horrid image doth unfix my hair and make my seated heart knock at my ribs" mean in Shakespeare's Macbeth?

In act one, scene three, Macbeth receives a seemingly favorable prophecy from the Three Witches, who tell him that he will be named the Thane of Cawdor and become the future King of Scotland. Shortly after receiving the prophecy, Ross and Angus arrive to inform Macbeth that King Duncan has just given him the title Thane of Cawdor. Both Macbeth and Banquo are astonished by the news, and Macbeth immediately begins thinking about ways to attain the Scottish throne.
In an aside, Macbeth contemplates the positives and negatives associated with the witches' prophecy. Macbeth then begins to imagine assassinating King Duncan in order to become king, which is a horrifying, unsettling thought. Macbeth describes his feelings regarding the nature of the prophecy by saying,

If good, why do I yield to that suggestion Whose horrid image doth unfix my hair And make my seated heart knock at my ribs, Against the use of nature? (Shakespeare, 1.3.137-140)

Macbeth is essentially saying that the thought of committing regicide makes his hair stand on end and heart pound against his ribs in an unnatural way. These comments reveal Macbeth's ambitious nature and conscience. Judging from Macbeth's aside, the "horrid image" of murdering King Duncan is evidently disturbing and unsettling to him at this point in the play.

Macbeth was told by the Weird Sisters that he would become Thane of Cawdor and King of Scotland, and then, lo and behold, Ross and Angus arrive, bringing the news that he has, indeed, been named Thane of Cawdor. Now Macbeth really begins to believe, in earnest, that the more significant prophecy could come true, and he begins to entertain ways in which it might. He says,

I am thane of Cawdor.If good, why do I yield to that suggestionWhose horrid image doth unfix my hairAnd make my seated heart knock at my ribs,Against the use of nature?

In other words, he is now Cawdor, as the Weird Sisters said he would be. However, if this is a good thing, he asks himself, then why does he now find himself considering the murder of the current king, which is such a horrible idea that it makes his hair stand up on end and his heart pound loudly within his ribcage in such an unnatural way? Macbeth is already considering the nearest way to the throne, and he is unnerved by how quickly he has jumped to regicide as a possibility.