Few clarifications first.
alt is not a tag, its an attribute which is meant to provide alternate text for non-textual elements to serve as content when the element cannot be rendered normally. (see HTML 4.01 Spec for more information.
The fact that IE displays it as a tooltip when user hovers over an image is incorrect behavior and should be exploited.
If you want to provide advisory information use title attribute
Most graphical browsers display the content of title attribute as a tooltip using OS functionality. You can not affect neither presentation nor behavior of these tooltips.