no, not quite. what I'm looking to do is to add a whole new class to the images in the HTML, not just manipulating what already exists with CSS.
For example, in the HTML, WordPress adds this to an image tag:
<img class="aligncenter size-large wp-image-129"...
What I want to do is to add another qualifier to the image class called 'tilt'.
You see, certain elements on the site (heads and images) get rotated slightly between -2 and 2 degrees. But the rotation is controlled randomly. I add a random number generator to the functions that control those elements that creates and adds a new series of classes between 'tilt0 and 'tilt5'.
I've managed to do this for all the other elements I need to, but I can't figure out the function that adds those classes ('aligncenter', 'size-large', etc.) to the images. It does appear that the get_image_tag function in the media.php file is the one that does it, but I can't quite figure out how to take control of it. And it doesn't help that there is absolutely no mention of the get_image_tag filter anywhere in the codex, and that other forum queries on the subject have received almost no response whatsoever.
If it makes any difference in the answer, what I'm working on is a child theme of Thematic.