About John Tenniel and his illustrations

John Tenniel as illustrator
Sir John Tenniel (1820 - 1914), an English illustrator and political cartoonist for the magazine 'Punch', made the illustrations for both Alice books.

He got precise instructions from Dodgson, so the Alice we all know is most certainly the Alice that Dodgson imagined. Tenniel never liked to take work from outside, and Dodgson had driven him almost crazy by providing him with so much details and instructions, so he almost turned it down when he was asked to illustrate the sequel. It was probably only his love of drawing animals that persuaded him to contemplate it at all.

The model
Alice Liddell was not the Alice of Tenniel's pictures. Carroll sent Tenniel a photograph of Mary Hilton Badcock, another child-friend, who was the daughter of the Dean of Ripon. He recommended her as a model, but whether Tenniel accepted this advice remains a matter of dispute. The following lines from a letter Carroll wrote some time after the Alice books had been published, suggest that he probably didn't:

Mary Hilton Badcock "Mr. Tenniel is the only artist, who has drawn for me, who has resolutely refused to use a model, and declared he no more needed one than I should need a multiplication table to work a mathematical problem! I venture to think that he was mistaken and that for want of a model, he drew several pictures of "Alice" entirely out of proportion - head decidedly too large and feet decidedly too small."

Still, the illustrations do quite resemble Miss Badcock

Creating the illustrations
According to Rodney Engen, Tenniel's biographer, his method for creating the illustrations of the Alice books was the same as the method he used for Punch, namely preliminary pencil drawings, further drawings in 'ink and Chinese white' to simulate the wood engraver's line, then transference to the wood-block by the use of tracing paper. Then the drawings were engraved to the highest standards, in this instance by the Dalziel Brothers. Carroll appears to have ordered many (expensive!) changes to them. The final stage in the reproduction process was to make electrotype plates from the wood-engravings, using them as masters. The electrotype plates were used for the actual printing. 

Because of the difficult process of creating wood-blocks involved, sometimes concessions had to be made as to the overall design of the illustration. For example, a character might be moved into a different position - which probably happened with the ape in the illustration of the Dodo with the thimble.
And, once wood had been removed, it could not be put back without a great deal of difficulty. A small number of Alice wood-blocks have had alterations or repairs made to them, that are in some cases detectable from the proofs which have been taken directly from the blocks. For example, the wood-block of the Hatter at the trial scene, the section showing the Hatter’s cup with a piece bitten out, had to be repaired and re-engraved.
(Source: Edward Wakeling's paper on John Tenniel)

In 1981, the original wood-blocks were discovered in a bank vault where they had been deposited by the publisher. They are now at the British Library.
(source: Jo Elwyn Jones and J. Francis Gladstone, The Alice Companion, 1998, p.252)

The following chronology of the illustrations of Alice in Wonderland is coming from Jones' and Gladstone's Alice Companion, 1998, pages 253-5:

25 January 1864:
Carroll asked Tenniel to illustrate Alice's Adventures in Wonderland
5 April 1864:
Tenniel consented. The fee agreed was £138.
2 May 1864:
Carroll sent Tenniel the first piece of slip set up for Alice's Adventures
12 October 1864:
Tenniel's first drawing on wood of the White Rabbit scurrying away from Alice was inspected by Carroll and 34 illustrations were agreed
28 October 1864:
The Dalziel brothers showed Carroll's proofs of several of Tenniel's pictures. The cost for the engraving of Tenniel's plates by the Dalziels was £142 for 42 plates
May 1865:
Carroll sent the galley proofs for all the text to Tenniel so he could complete the illustrations. Forty-two illustrations were completed
June 1865:
The Clarendon Press, Oxford, printed 2000 copies of Alice's Adventures at a cost of £131
20 July 1865
Tenniel objected to the quality of this first printing and Carroll rejected it
November 1865:
Richard Clay, the new printers, achieved an edition which satisfied Tenniel and Carroll. Carroll proposed to employ them again if he wrote a second Alice
Carroll wrote to Alice that, including the People's Edition and the first translations into foreign tongues, 120,000 copies of Wonderland had sold
8 April 1868:
Carroll reported Tenniel's warning that there was 'no chance of his being able to do pictures for me until the year after next, if then. I must now try Noel Paton.'
19 May 1868:
Noel Paton urged Carroll to persist with Tenniel. So did Ruskin. Carroll, in desperation, offered to pay Punch for his time 'for the next five months' to free him to illustrate the second Alice
18 June 1868:
Tenniel made what Carroll described as a 'kind of offer to do the pictures (at such spare time as he can find)'. Tenniel hoped the illustrations would be ready by Christmas 1869
12 January 1869:
Carroll sent the first chapter of Looking-Glass to Alexander Macmillan
20 January 1870:
Carroll saw the first ten Tenniel sketches for the pictures of Looking-Glass
12 March 1870:
Carroll and Tenniel met for two hours in London to set out the plans for 30 more pictures, having already sent three to the Dalziel Brothers at Camden Press for 'cutting'
4 January 1871:
Carroll finished the manuscript of Looking-Glass
16 January 1871:
Carroll sent the completed galleys, including the Wasp incident, to Tenniel for pasting up and illustrating
March 1871:
Carroll moved the picture of the Jabberwock to the text pages and substituted the White Knight as the frontispiece
25 April 1871:
To this date, Carroll only received 27 pictures. Tenniel now hoped to complete them by July
21 November 1871:
Carroll sent authorization to Clay by telegraph to electrotype 'all the rest of the Looking-Glass. I afterwards sent two corrections by post. So ends my part of the work.'
30 November 1871:
Macmillan advised Carroll that they already had orders for 7500 copies: 9000 were to be printed and a further 6000 were ordered
6 December 1871:
Carroll received the first copy of Looking-Glass
15 December 1871:
Carroll sent the Dalziel brothers a cheque for £203.16 for the engraving
27 January 1872:
15,000 copies of the story had been sold
Tenniel agreed to supervise the colouring of 20 illustrations for The Nursery Alice. The book was colour-printed by Edward Evans and the cover was drawn by Carroll's friend and life-drawing teacher, E. Gertrude Thomson.

For the 'The Nursery Alice', 20 of his illustrations were enlarged, colorized, and some of them were even slightly redrawn. Among others, Alice's dresses were drawn with less crinoline. Dalziel's signature has been removed from all Nursery illustrations.

Some doubt has been expressed as to whether Tenniel was personally responsible for the coloring of the illustrations to The Nursery "Alice", largely because of the advertisement which appeared in the 1886 facsimile edition of Alice's Adventures Under Ground (and later in the 1887 'People's Edition' of Alice) that announced The Nursery "Alice" as "in preparation": "Being a selection of twenty of the pictures in Alice's Adventures in Wonderland enlarged and coloured under the Artist's superintendence, with explanations." It seems likely, however, that this simply refers to Tenniel's supervision of Edward Evans' colour printing.
source: Brian Sibley, "Jabberwocky", The Journal of the Lewis Carroll Society, Autumn 1975

Carroll recorded in his diary on 29 March 1885, that twenty illustrations for The Nursery "Alice" 'are now being coloured by Mr Tenniel', and by 10 July he was able to report that 'Mr Tenniel has finished the coloured pictures for The Nursery "Alice"'; although, in fact, the author was not to start the text for another three and a half years.
source: Brian Sibley, "Jabberwocky", The Journal of the Lewis Carroll Society, Autumn 1975

Tenniel's drawings remained black and white for over 40 years until 1911, when eight prints in each book were hand colored.