I've looked at this before to use .png format icons for transparency and larger, high-contrast icons for accessibility reasons. As far as I am aware it's not easily done at the moment.
There was some work done on the sideblocks recently, but looking at my most recent CVS version there doesn't seem to be any hooks for attaching images via CSS. Maybe it has changed in the last week or so, my copy is out of date.
(If you knew all the students were only going to access Moodle using the Firefox browser you could use some advanced CSS to do this, but that solution would be as much work as rewriting the PHP and offer less cross-browser support.)
I think this needs to be rewritten anyway (I believe Martin's mentioned the need to move images out of the HTML and into the CSS), so if you did the work you could maybe check it into the Moodle 1.7 development branch for collaboration and feedback. I've got some code lying about that shows how I approached the problem.
You can see it in this post though note that the demo links don't work anymore.