In Transparent image background in beamer there is a nice example to find showing a background image with text in front of it.
I would like to show the picture in a first step and "dim it down" (e.g. with a partly transparent white block in front of it) in a second step to show some step. As far as I know (a test showed it, too) it is not possible to use \visible out of the frame-environment.
What do I have to do?