It was already installed, but - after I closed the Extension Manager 5.5 the 'enabled' checkbox was checked. So I think the extension is installed. But there was nothing to see in DW.
I open the Extension Manager once again, what happend, the checkbox is not checked, - I don't know why? So I checked it again and close the Extension Manager. I start DW and it does work.
Thanks for your reply.
PS: I can only post in OPERA. The 'Image Verification' is not displayed in Safari, Firefox and Chrome.