Dickens worked in close collaboration with his illustrators, supplying them with an overall summary of the work at the outset for the cover illustration which was printed on heavy colored stock, usually green, which served as a wrapper for each of the monthly parts. Dickens briefed the illustrator on plans for each month's installment so that work on the two illustrations could begin before he wrote them.
This close working relationship with his illustrators is important to readers of Dickens today. The illustrations give us a glimpse of the characters as Dickens described them to the illustrator and approved when the drawing was finished. Film makers still use the illustrations as a basis for characterization, costume, and set design in the dramatization of Dickens' works.
When Robert Seymour committed suicide after the second installment of Pickwick the author and his publishers needed a new illustrator. Artists such as John Leech, William Makepeace Thackeray, and Robert W. Buss were considered but the man selected was Hablot Knight Browne who had done some work for Chapman and Hall earlier and had worked with Dickens on a recent pamphlet.
Browne and Dickens developed an excellent working relationship and Browne took the nickname Phiz to complement Dickens' Boz. Browne would go on to illustrate Dickens' work for 23 years, ten of Dicken's novels were illustrated by Phiz. Browne's comic/satiric style of illustration did not fit well with Dickens' later, more serious, novels and after the somewhat disappointing illustrations for A Tale of Two Cities, he never worked for Dickens again.
Phiz and Emblematic Detail
In the background of many of the Phiz illustrations of Dickens' novels the illustrator introduces details that help to interpret what is happening in the story. Some of these emblematic details are rather obvious and some are more subtle. Michael Steig, in his book Dickens and Phiz, argues effectively that, although Dickens gave detailed instructions as to the content of the illustrations, many of the emblematic details in the illustrations were added by Phiz on his own.
An excellent source of information about Dickens illustrators (including the above) can be found here.