The success of your portrait depends on your utilizing positive AND negative space in an effective manner. But what are these two things? POSITIVE space is the 'actual stuff' in a picture, NEGATIVE space is the 'white' or inactive space with 'nothing' in it. Sometimes less is more, and you can say a lot with 'nothing.' Such is the case here.
Another way to think about positive and negative space is in terms of 'FIGURE/GROUND RELATIONSHIP', one of the components of Gestalt theory listed on pages 126-127 in Basics of Design. Our dingbat project touches on other components of this theory as well: PROXIMITY and SIMILARITY, which both essentially say that if you group stuff together, people will see it as a whole object--ie, like the DINGBATS we are using in the positive space areas of your portraits.
Here are some links to check out:
http://www.artsconnected.org/toolkit/watch_space_positive.cfm
http://www.artsconnected.org/toolkit/create_space_positive.cfm
http://en.wikipedia.org/wiki/Negative_space
POSITIVE--NEGATIVE SPACE / FIGURE-GROUND RELATIONSHIP
PROXIMITY AND SIMILARITY
