Hero-template with parallax

hi there,

while trying out the hero.html.twig that is included in the quark theme i got some problems to get it shown up properly.

whatever size i tried as image it appears with the bottom part of the image at the top and the upper part at the bottom. so it shows up in a wrapped way.
… hope this was comprehensible …

Could anyone give me a hint on what i missed?
Is it necessary to use images with a special size or ratio

thank you for any help