Illustration or Design?

I am required to write a reaction paper about one of the artist lecturers we met in Texas last week. I have it narrowed down to two very talented artists, Linda Helton (b. 1959) or Bart Forbes (b. 1939). The images above are by Helton, created in traditional media. Though their work is very different from each other, both artists appear to be succeeding in today's tough market.
During her presentation, the work Helton showed us varied in style, technique, and media. It is colorful, upbeat, and appealing to a wide range of markets. It was fantastic. I was impressed by her work ethic and attitude toward being a freelance artist. In researching her online, I've found she's done a wide range of stock artwork. Each image is simple, beautifully designed, and could be used in many different ways. In the last five years I have watched the commercial art market change rapidly. I find that I have to reinvent myself and approach to the industry. I wonder how this stock artwork approach is working out for artists.
I also find myself asking what makes an image an illustration or when does it become a design? Does it depend on how much of a narrative theme a picture contains? On how simple or complex the image is? I hear some of my friends describe themselves as designers rather than illustrators even though they draw pictures of living things, sometimes in the context of a story. When do these images stop being designs and become illustrations?
At this point I am seriously putting off work. I will have to ponder these issues after my paper and thesis are done and I am posing at Starbucks with my artist peeps.