Image Resize for thumbnail pictures in listings

I’ve already searched the forum for a solution but didn’t find one. I don’t have a problem with image resizing itself. The problem I have is that the thumbnail image in a listing is just downsized. So the complete image loads in the background but it’s just scaled down.

You can see it here: https://www.raceandtravel.de/listing/oschersleben/
That thumbnail and all other pics are being loaded it their original resolution.

The results on Google PageSpeed Insights speak for itself: http://www.raceandtravel.de/image-resize-page-speed.png

The problem gets even bigger when you upload ten images in a better resolution for visitors.

Does the theme not generate thumbnails for these pictures? For the avatar and futher entries (Weitere Einträge) on the left too?

Thanks
Dennis