Hi Shell and welcome from New Zealand. Don't worry too much about where the light source is for teh small images we use on cards. To shade darker around the edge of the image and ligher in the center, as you are doing for faces, is perfectly acceptable. With smaller parts of the image like hands or boots, I wouldn't worry about the light source at all. Just colour them uniformly with one shade. Too much shading detail is just confusing on a small image anyway.
With your promarkers, I would suggest that you initally just play with them. See how they behave on different paper types and surfaces, play with shading or blending, stamp several practice images and use the markers to see how they go without worrying about producing anything. Have fun with the process rather than think you have to end up with a 'product'.
To live a creative life, we must lose our fear of being wrong. Joseph Chilton Pearce