You're wellcome the next days I will upload a picture.
Now an update: The down dimming og pictures I've got not with PDF and PPT over OpenLP. These same phenomenon appears by working with Libreoffice, too. A picture opened with Gwenview produces the same result. I know not much but a little step. The next church service seems to be saved, gg.
It has been chanced no settings on the beamer. With a Win laptop everything is good.
Maybe I should take this topic to a Ubuntu forum.