The reading used for 挿絵 is an exceptional one. Instead of そう it's さし. When you think of a book illustration, think of an illustration of a sash. Just picture after picture of a sash. Purple sashes. Green sashes. Flashy sashes. What a weird book, that only has illustrations of sashes.
絵, however, is the reading you learned with the kanji, so at least you have that.
According to him, she draws illustrations for books.