Today's the "most important day" in Mark Pilgrim's accessibility series. The topic: text equivalents for images. In short, he says every image needs an ALT attribute.
This is an oldie but goodie, relatively well known compared to some of his other tips, but that doesn't make it less important.
Something I've found useful (and that Mark didn't mention) is the "ALT attributes -- show all" favelet on this page. It's a tiny, bookmarkable script that gives you a list of all the images on whatever page is open in your browser, along with their corresponding ALT values. That way, you can easily weed out the images that don't have 'em.
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