Hi Jenny,
Because the royalty on a picture book is split between the author and illustrator some agents are less interested in repping picture books from pb writers because of the smaller royalty percent. However, if you are an author/illustrator you may have better luck and find this isn't an issue for you in dealing with potential agents because your agent would be repping both halves of the pb royalties. Ultimately you have to decide whether or not you want an agent, but I wouldn't let what you read deter you if that's what you want.