Extending the Narrative.

In the image of Little Nell's death the narrative is extended through metaphors by (in a paradox) altering the narrative of the text. The illustration is contributing to the continuum of the narrative and so alters our reading experience.

The placement of the illustration within the text can also extend the narrative and change our experience of the reading the book. 

On little Nell:

"Coming at the end of the chapter, it provides us with a moment of stati, of contemplation, in the face of the death f the character. That the engraving is placed sideways in the original edition, with the consent and to the satisfaction of Dickens, is also significant. We have to turn the book around to take in the image, and in doing so we suspend the continuum of the verbal discourse. The presentation of this moment of stasis within the text is only one function of the image, which also extends and deepens the verbal text in ways explored above." -Stuart Sillars

The placement of an illustration therefore is almost as important as that which it depicts. You may want pauses in your text for contemplation, to build tension ect. but you do not want to give away too much at the wrong moment. If the illustration of Little Nell were to come before the end of the chapter interrupting the text, then you'd be forced to stop reading unnaturally and wouldn't necessarily have the moment of reflect or perhaps even the understanding of what is presented. Furthermore if an illustration with such obvious connotations is shown before the event in the text this would alter our reading experience for the worst, ruining the 'ending.' Spoilers if you will.

The placement of illustrations can have a number of functions including heightening the suspense of a scene by delaying a moment in the narrative. Advancing the narrative by restraining it. An illustration may extend the text in its foreshadowing of events, depicting a happy scene like a weeding with signifiers of a funeral ect. evokes a sense of dread or unease. The illustration almost allows for a kind of dramatic irony, in which the reader knows more than the character.