I figured it out:)
In the file jquery.nivo.slider.pack.js, just after the part that starts with processCaption
I added:
if(settings.captionHide){
$('.nivo-caption', slider).hide();
slider.hover(function(){
$('.nivo-caption', slider).show ();
}, function(){
$('.nivo-caption', slider).hide ();
});}
the at the very bottom, were the defaults are, I added
captionHide:true
In the css I added cursor:pointer to the .nivo-caption also.