That’s one of the questions I had never asked myself before I started reading extensively on the topic of using visuals in e-learning. I used to think that it was always good to have a picture or a diagram next to any text, and it was the cost and creativity required that prevented me from uploading hundreds of graphs and photos to accompany everything I wrote. Now things have changed: it is great to be able to justify your decision to add or not to add graphics.
So far I have read two books on the topic, but both of them are good.
Graphics for Learning by Clark & Lyons takes more of a HOW TO approach and is well illustrated.
The Cambridge Encyclopedia of Multimedia Learning is a great handbook when it comes to WHY you should or should not combine certain media in a certain way. I was surprised to discover about two dozen different principles there: the editors did a very good job getting everything described in such a concise and readable manner. The fact that I did not fall asleep every other chapter while doing the initial flicking through does mean something: it is difficult to make me read encyclopaedias as enthusistically as I read whodunnits.