Not builtin, because there as many ways of doing a rollover effect as
there are web authors... You can always insert an img element of a
given height and width, apply a css background through the css panel,
and apply another background on :hover.
All of that will be replaced in CSS 3 by the 'content' property applied
to all elements, including images:
img { content: url(firstimage.png) }
img:hover { content: url(secondimage.png) }
</Daniel>