Correction ...
The alternate text, ALT, is supposed to show when the image can't be loaded, not when it can. It was IE that came up with the bright idea to display them as tooltips.
TITLE on the other hand should be displayed. In what way isn't specified, but guess a tooltip is the only realistic way. It can be applied to most elements and isn't an alternative to ALT as one might believe. It's perfectly OK to have both ALT and TITLE in an IMG tag. Even IE will display TITLE if both are present.
End of HTML lesson.