Illustration covers a broad spectrum of areas. In order to make a living, illustrators often had to diversify. Some created postcards, some were lucky to have their illustrations grace the cover of popular magazines, others developed comic strips and the variety goes on and on...
You may not be familiar with Charles Twelvetrees work but his illustrations were very popular in the early 20th century. He developed comic strips like Johnny Quack:
He also created many postcards and greeting cards. His work primarily featured children. Long before Anne Geddes made a name for herself photographing babies in baskets and other unique settings, Charles Twelvetrees had created babies peaking out baskets and barrels:
Twelvetrees did a lot of illustration of baby girls scantily covered which I guess was acceptable back then but seem strange to us now: