Hello, Florian
Please read the next articles – https://xstore.helpscoutdocs.com/article/89-product-images https://woocommerce.com/document/fixing-blurry-product-images/
“I also noticed that several pictures are blurry/unsharp (mainly on desktop).” – please provide screenshots of such pictures and URLs of pages with them, because we do not see this problem on your site.
Also, you may provide temporary wp-admin access, we will take a closer look.
Kind Regards,
8theme team