The illustrations Horton Hears a Who! are done in Seuss's usual style: plenty of wildlife with strangely humanlike qualities. The book centers around a place called the Jungle of Nool, and Seuss makes it a character in its own right: we see lots of lush jungles, mountains, and even a field full of millions of fluffy pink clovers.
The muted colors in the book go hand-in-hand with the emphasis on the topographies of the Jungle of Nool and Who-ville. From the very start, the Whos are a mysterious race, their presence unseen and unknown. Thanks to the magic of Seuss's illustrations, though, we can see what no one in the book can. We can see their extravagant little town and all the folks who live there, going about their everyday lives. We can see it all in great, cartoonish detail, while no one else sees anything but a speck.
It should come as no surprise that Seuss focuses a lot on everyone's expressions. For example, when Horton is bullied, he looks awfully frazzled; the kangaroo is often standing with a haughty expression on his face and his arms crossed; the Whos look pretty worried when they find out they might be boiled (go figure); and Seuss is careful to let us in on all those emotions.