I think the key to good photos/selectable icons is consistency. If you are using photos of the actual model, then take them all under the same lighting. I take all mine outdoors under natural sunlight. If you are using computer generated images, don't mix those with pictures of models.
If you do happen to use pictures of your models, good lighting is key. Hence the reason I take mine outdoors. Also use a neutral background that is easy to remove on Photoshop or similar programs. Tripods are very handy and recommended for any model photography.
-Kevin