What is the info box called that pops up when you mouse over an image? How do you go about implementing it?

It's an ALT Attribute

<img src="http://www.domain.com/images/logo.gif" alt="Some Text Here">

I found out the solution. The alt attribute will not work in FF, but the title attribute will. I just don't know if the title attribute works in other browsers, or if I have to use both the alt and title attributes.

Oh, and the little box is called a tool tip.

The title attribute will work in almost any (x)html tag in all moderen browsers. The alt attribute is used only in the image tag, and only shows up in IE. The alt attribute is supposed to display when the image src is broken or cannot be downloaded/displayed.

yep the title attribute is cross browser, and if you want to suppress the incorrect IE behaviour of showling the alt attribute just use a blank title (title="")