Many people are confused about the difference between an alt attribute and a screen tip.
The alt attribute is for accessibilty so that those who are using screen readers or who do not have the ability to view images. Many people who are on bandwidth limited connections such as cell phones and Pocket PCs connecting over a cellurar connection will turn off images so that pages will load quickly.
Internet Explorer 6 and below have incorrectly implemented alt attributes so that when the visitor mouses over the image the alt attribute value will “pop-up” like a tool tip. This error has caused many designers to use the alt attribute value as a tooltip.
Firefox, Safari, Opera and other browsers that are more standards complaint browser will not render the alt attribute as a “tool tip”. Instead the contents of the title attribute wil be rendered with or without an alt attribute.
In Expression Web you will need to add the title attribute i code view so that it looks like this:
title="Download Expression Web" /></a>
Which will display like this: