There is not much happening there. Basically I am using the default Quark base.html.twig for general layout (loading css & js as well as defining navigation). And this is what I am using inside my template:
<div class="box">
<div class="scroll-pane">
<div class="scroll-content">
<div class="padding">
{{ page.content|raw }}
<p><a href="#" class="button5">More info</a></p>
</div>
</div>
</div>
</div>
Plus this piece of Markdown as content:

# About me
Firstname Lastname
Artist & Psychologist
I have a high resolution display but if I resize the image with other online-tools (even tried with ImageMagick) the result is always good and not blurry. So I assume there must be some kind of setting. If it helps I can post the results here as well as images. And it does not only happen with png but also jpg.
Any help/hint is highly appreciated.