For meeting Web accessibility standards and writing valid HTML, we must always populate Title or Alt attributes for HTML Image tags (even if that means an empty string like alt="" or title=""). But most of us are not too sure of what to put in the TITLE and ALT attributes of the IMG tag. We sometimes omit them completely or use both of them to confuse the screen readers. Here's some advice:

Jon Cram writes that alt is for alternative text. What you want to think here is: “What text should be displayed instead of the image such that the user experience is as unchanged as possible?”.

For example, if you had an image showing you and your family on a skiing trip in the Rocky Mountains in December 2005, your alt text might read: “Family skiing trip, Rocky Mountains, December 2005?.

Like Jon, Roger Johansson writes that Alt text is an alternative, though Internet Explorer will continue to display alternative text as a tooltip.

John also suggests avoiding title attribute for image and prefer using them for hyperlinks instead. The title of a link helps the user decide whether is it worth to click the link. In this respect, your title text needs to describe what the user should expect if they click the link.

According to Matt G, IE uses alt attribute to display tool tips style descriptors while FireFox uses alt attribute. He would therfore use both the attributes for IMG tags.

I would agree with Roger here: Only use the alt attribute to specify alternative text. If you want to provide supplemental information that browsers may choose to display as a tooltip, use the title attribute.