Adding a new picture to the appstore needs a new build of the app
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.