Don't use ALT= for your images according to this bug on Mozilla.org.

I came across this 'bug' in Mozilla a few weeks ago, when I was coding the new work website. If you specify ALT text for your images, it will not display as a tooltip.

This was a conscious decision on the Mozilla dev team, to interpret the W3C recommendation very strictly.

If you want alternate text to display as a tooltip for the image, you must now use the TITLE= attribute. There's a downside to this, of course; TITLE is not supported by Netscape 4.x. Of course, since all good web developers adhere to HTML/CSS standards anyways, this isn't a problem, right? :)