Welcome, Will! 
The issue lays here:
<div class="topic-image" style="background: url(https://forum.cyberbrein.nl/uploads/default/original/1X/aef8c5e27bb1320fd8da2319220c5193e56e2323.jpeg);background-position: center;background-repeat: no-repeat;background-size: cover;"></div>
Remove background-size: cover;, or add:
.topic-image {
background-size: auto !important;
}
in your theme or a new component if you can’t edit the component that creates these thumbnails.
edit: it seems your use this theme: FKB Pro - Social theme
I’ll move your topic in the dedicated topic. 