Adding a new picture to the appstore needs a new build of the app It seems to be true, that I have to build a new app-version for only adding a new picture in the appstore. So my App has the same code, but a new version and I don't know what to write in the update text for the users. Its stupid to write "I uploaded a new picture, thats why you guys have to upgrade the app ".

Is this just a design glitch again from Apple, or is there a real reason for that ?
It was a slightly common tactic for spammers to change the screenshots to mimic a different app. Hence, from 2013, you are required to create a new app version to change screenshots.

Ideally, you should not update your app version just to change the screenshots — perhaps you can wait until you really have a new version to upload?

You can read more about this functionality at App Store Screenshot Changes - MacStories.