The alt="" attribute has ALWAYS been used WRONGLY as a tooltip in IE and some other browsers. The real tooltip attribute is title="". Use the "title" attribute when you want to have a standard popup tooltip on mouseover.
Never use the "alt" attribute as a tooltip as it is used for accessibility instead.
There are some fancy JavaScript tooltips which are way more fun to use as you can customize them:
http://ashishware.com/Tooltip.shtml
http://willcode4beer.com/tips.jsp?set=bubbleTooltips
http://www.javascriptkit.com/script/script2/tooltip.shtml
http://www.walterzorn.com/tooltip/extensions.htm
http://www.freejavascriptkit.com/free_javascripts/tooltip_hint/dhtml_mouseover_tooltip.html
http://www.dynamicdrive.com/dynamicindex5/dhtmltooltip.htm
http://www.dynamicdrive.com/dynamicindex4/imagetooltip.htm
http://boxover.swazz.org/
http://www.xinod.it/scripts/tooltip_gallery/
http://www.walterzorn.com/tooltip/tooltip_e.htm
Ron