The primary illustrator of the Joey Pigza books is Beata Szpura. (She illustrated all of the harcover editions). Check her out at http://www.bszpura.com/.
Other illustrators have contributed different covers for some of the paperback editions, including Brian Selznick (this years Caldecott winner for The Invention of Hugo Cabret)who illustrated this paperback version of Joey Pigza Swallowed the Key.
Here is another paperback version by Cliff Nielsen. Which is your favorite cover?