OpenOffice.org Impress: Using Master Slides

“It’s all a matter of contrast, really — which is why a
background of a full-color photo is usually a poor choice for
readability. No matter what text color you choose, some part of a
background photo won’t contrast with it. And, given that the
perversity of the universe is set on stun in such cases, the text
that blends into the background will probably be the text you want
to emphasize the most. If you must use a photo, try to position it
on the edges of the slide or in a footer (see below), where it is
less likely to conflict with text.”

Complete Story