In most cases it would be easier for cross-browser compatibility to create a seperate image, that appears to be transparent (ie, throwing a 30% alpha'd white square over the origional image in photoshop or fireworks) and set it as the background of the scrolly news thing (if that's what you're referring to.
Alternately some browsers will allow the use of png's that have alpha (ie. firefox), IE does not allow transparent png's as of now. Absolutely positioned divs with transparent png backgrounds make creative and interesting effects. The only drawback is the inability to click through a transparent png -_-