In this passage, Lady Macbeth is chiefly attempting to quiet her husband and to impress upon him that no one has actually witnessed the murder he has committed. Macbeth is terrified that his killing of Duncan will be discovered. Both this fear and his sense of guilt are causing him in effect to hallucinate, as he will continue to do as the action of the drama develops. A voice has been heard crying that Macbeth has "murdered sleep" and that he shall "sleep no more." In Macbeth and other works of Shakespeare, it's often an open question as to whether such perceptions by people in a hyperemotional state are in fact illusory or are, rather, supernatural occurrences. But whatever our interpretation, it's clear that Lady Macbeth wishes to dismiss her husband's fears and that she's impatient with him and even slightly disgusted with the terrified manner in which he has reacted to the situation. By saying that the sleeping and the dead are mere pictures, she's implying that they can't hurt him any more than a lifeless picture or image could do.

The more significant point she seems to make, however, is that not only is there an effective likeness between sleep and death based on the absence of awareness, but also that in her view, death is no worse than just a kind of sleep, albeit a perpetual one. Lady Macbeth fears nothing, essentially because she believes in nothing. As with Edmund in King Lear, nature (i.e., the natural or material world) is her "goddess." The idea that the dead will live forever is foreign to her: death is merely a physical state which has no more importance than the harmless condition of not being awake. Her mindset dictates that killing someone is, for all intents and purposes, no worse than just making that person go to sleep, and Macbeth should see himself not as a guilty party in the manner that religion and morality would declare him to be.

If those who sleep and those who are dead are mere "pictures," one has nothing to fear from them. But a further implication of the simile is that the world itself is a giant illusion, an image created by our minds, as many philosophers have viewed it. In some sense this fits in with Lady Macbeth's amorality, for if the world and the states of being (life, death, sleep) in which people find themselves are illusory, then presumably an intentional killing is just a meaningless act. Yet the guilt in which Lady Macbeth herself is eventually submerged indicates that she doesn't actually accept this and that she believes she and Macbeth will be punished for the murders.

It is also worth noting the foreshadowing of Lady Macbeth's demise later in the play with these words.  She will be the one, ultimately, who is haunted by pictures of "the dead" and she will replay her nightmare of killing Duncan while "sleep"walking.

Shakespeare is full of these reverberations, and it is astonishing when you consider the connections that exist between seemingly simple lines of text and actions and words spoken by other characters in the play.

Later, in Act V, Macbeth seems to have taken on the point of view expressed in the lines you quote when he says:

...Out, out brief candle.

Life's but a walking shadow, a poor player

That struts and frets his hour upon the stage

And then is heard no more.

He is taking this idea that the dead are just pictures one step further to suggest that death equals nothing.  He is also making a nice nod back to Lady Macbeth's sleepwalking with the images "out brief candle" and "walking shadow."  And the idea that those who are asleep or dead have no reality is actually extended here by Macbeth into the notion that even those who are "strutting" their "hour upon the stage" of life also have no ultimate purpose.

Shakespeare truly does provide layers of connection and meaning, and the seemingly simple lines you quote are good example of this.

What Lady Macbeth is trying to say here is that both dead people and those who are sleeping cannot actually hurt a person.  In this way, they are both like pictures.  This is why she starts this passage by telling her husband that he is "infirm of purpose."

Basically, she is telling Macbeth that he is like a little kid.  He is allowing things that are not real -- things that are just like pictures -- to scare him.  He needs, instead, to be a man and not be afraid of things that can't hurt him.

So what they have in common is that they are not real and cannot hurt anyone.