According to him, she draws illustrations for books.
If you insert a drawing into a book, it becomes a book illustration.
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.