That's actually as required by the standard - IE is the only browser which displayed the alt text in a tooltip. I expect if you switch it to IE7 compat mode it will show.
"I noticed that image tooltips were missing in IE8 and Firefox for all web pages I had designed. I then realised I had only specified alt attributes instead of title attributes!
Having a portfolio of hundreds of Web pages I was faced with the time-consuming task of adding title attributes to applicable images. Thankfully I stumbled upon a Windows application called Alt&Title that did this tiresome task for me!
Instead of spending hours manually adding title attributes, Alt&Title did it for me in minutes.
Give it a go. I hope you find it as useful as I have!
