Shel Silverstein's poems are illustrated by himself. The illustrations often provide the 'punchline' of the poem, as in the following examples (all taken from Falling Up. Pictures are mine - feel free to replace them with better ones):

Safe

I look to the left,
I look to the right,
Before I ever
Move my feet.
No cars to the left,
No cars to the right,
I guess it's safe
To cross the street...

This poem is accompanied by this illustration:

This is a safe falling out of the sky onto the girl's head.

Unfair

They don't allow pets in this apartment.
That's not decent, that's not fair.
They don't allow pets in this apartment.
They don't listen, they don't care.
I told them he's quiet and never does bark,
I told them he'd do all his stuff in the park,
I told them he's cuddly and friendly, and yet -
They won't allow pets.

This is accompanied by this charming animal:

Web-Foot Woe

Us swans and geese
Have rotten luck.
You folks don't know
Whose name is whose.
I waddle in -
You all yell "Duck."
Can't you see
That I'm a goose?

With this pointed picture:

Imagining

You're only imagining
A mouse is in your hair.
You've got to stop imagining
That mice are everywhere.
I think you're just imagining
To give yourself a scare,
But trust me dear, I wouldn't lie:
There is no mouse up there.

With this slightly surprising picture:

I've made the point; there are more examples. So can these illustrations be considered part of the poem itself, as they are necessary to understand the poem?