You are mixing up terms and terminology.
What you are probably remembering is that when you 'mouseover' (an action,
not an event) an image with an ALT tag in Pub 2000, you can view the ALT
tag, but this does not work in newer versions. Furthermore, this only worked
in IE and not FF. You can also 'mouseover' an image and get a popup image
and other effects/events depending on the script you use.
While I believe using a text caption next to the pictures is by far the best
choice...the most viewer friendly choice, you can also use script that will
show a tooltip when you mouseover the images. Google for 'javascript
tooltips' and try out some of the free code you get.
DavidF