I have worked with the following freelance artists and am always on the lookout for more. As a customer, you would choose the illustration style that you most like and that matches the style and audience of the book.
Ali Page uses pen-and-ink, water color, and pastels for a look that is whimsical.
Kim Miranda is a digital artist with a light and compelling style.
Lynne Hull is also a digital artist whose portraits span from the cartoonish to the realistic.
Rob DiFiore hand draws custom photo-realistic portraits in pencil (monochromatic or prismacolor).