In Shakespeare's Macbeth, when Lady Macbeth tells Macbeth that he is infirm of purpose in Act 2.2, she means that he is not solid or concrete, that he is not firm, in his determination to go through with what he wants and needs to do. This is an insult. She is berating him, because he's afraid to take the bloody daggers back to Duncan's chambers. Of course, Macbeth was also an idiot for bringing the murder weapons with him in the first place.
She continues to berate Macbeth by making fun of him in other ways for not being willing to go back to Duncan's chamber to return the daggers. She ridicules him by saying that a dead body is just like a picture: harmless, of course.
And, she says, only a child is afraid of a painted picture, even if it is of a devil. That's the eye of a child and painted devil part.
Not only does Macbeth foolishly bring the daggers back with him to his wife, but he is afraid to take them back once she discovers them. And she berates and ridicules him for it.
These lines are spoken by Lady Macbeth in Act II, Scene 2 of the play. What is going on here is that she is scolding her husband. He has just killed Duncan and he is sort of freaked out about the fact that he has done this. This is where, for example, he thinks he hears voices accusing him. He also can't bring himself to pray.
So then Lady Macbeth says these lines. "Infirm of purpose" means that he's a coward who can't be brave enough to do what he means to do.
Then she says that people who are sleeping or dead cannot hurt him anymore than pictures can.
The part about the eye of childhood doesn't mean anything without the rest of the speech. She goes on to say that only a child fears a "painted devil."
So basically she's calling him a scared little child...