It works now.
As I can see, the image on particular event is normally displayed. For example, http://www.itech-progress.com/events/treffen-sie-uns-auf-der-oop-2015-responsibility-building-reliable-environments/
I know how to get to specific event, but how do i get to this page? http://www.itech-progress.com/events-uebersicht/
I believe this comes from WordPress image setup. Notice that the image is 300px x 300px in dimensions.
If you go to “Einstellungen – Medien” and check the “Mittlere Bildgröße”, you will see this dimension. Try setting it to 550px x 550px.
After that, you will have to regenerate thumbnails. You can do this in a very simple way – by installing a plugin called Regenerate thumbnail (https://wordpress.org/plugins/regenerate-thumbnails/).