Pictures for Linear, and Linear Artistic, PhotoStitch work best when they have strong contrast on the facial features, and no areas of shadow on the face.
Photos with face to camera tend to work better than those taken from the side, as the other side of the face is in shadow.
A pale background is often better than a dark background. Try removing the background in mySewnet™ Draw & Paint, or other picture editing software. Save the picture before making the PhotoStitch.