The speaker of this poem starts right away with a challenge to the power that Death thinks it has. He then goes on in each of three quatrains to give a specific reason why Death should not be so full of pride and arrogance. In the second quatrain, he says, "From rest and sleep, which but thy pictures be, / Much pleasure". It is easier to consider what he is saying if we keep reading the sentence. Sleep as a kind of death, or death being just a kind of sleep is a stock metaphor that has existed in the tradition of poetry forever, and Donne is throwing that metaphor directly in Death's face. He is saying, 'hey Death, you are nothing but a kind of sleep -- and most people like to sleep.' By suggesting that sleep (and by comparison, death) is a pleasure, it takes the power of Death away. He finishes the second quatrain with further explaination of the pleasure that comes from death when the soul is delivered from this earthly world, to an afterlife in Heaven.
John Donne's "Holy Sonnet 10," better known as "Death Be Not Proud," attacks in a series of paradoxes the conventional depiction of death as man's invincible conqueror. The poet tells Death, for instance, that he is not "Mighty and dreadful." Furthermore, he tells Death that he cannot "kill" him. The line "From rest and sleep, which but thy pictures be" indicates the poets refusal to accept the "resting" body of the dead as a final state. "Rest" and "sleep" are merely images; the soul lives on against the tyranny of an proud but ineffectual being, Death. For, this tyrant may be able to have dominion over the mortal body that "sleeps," but it cannot destroy the soul as well as the memory of the person's life.