If you have clip art images in a metafile format (such as WMF), they can be enhanced by putting a small drop shadow behind them (it only requires about 7 pixels, or so). Here's an example that I have used for my essay page:

and the unshadowed version:

What do you think? Which is better? Maybe it would be better to look at the page in which it is used. Oh, and remember to scale the image down before you add your shadow. I've converted these ones from a 200 pixel width to a 100 pixel width, and then applied the shadow.