Explain Lady Macbeth's simile :"The sleeping and the dead/Are but as pictures" (2.2.52-53). In her mind, what do the sleeping and the dead have in...

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.