I could solve the problem for me, maybe this solution will work for one of you too.
With WordPress 2.7.1 I’ve had no problems adding images to a post or page, but with 2.8.1 it did not work anymore. The problem was the AdBlock Plus plugin that I’m using with Firefox/Icewesel. When I deactivate the plugin for my blog and reload the page, the button for adding images works again.
Now I just need to figure out why I do not get any thumbnails for my images …
HTH
