The way I understand it- if the image is a photoshop or similar file with an "alpha" channel, just use the "source alpha" setting in the compositing section. If not, use chromakeying to make the background transparent...
I always use Photoshop and it works well, just put the image or text you want on top of a transparent background, don't flatten it, just save it as a PSD file and import it directly into Vegas.