Thumbnail quality reduced at frontpage

Hey there!
I’m having a problem with thumbnail quality for a post on my ghost selfhosted instance.
This is how it looks at frontpage:

After clicking on blog post quality increases greatly.

What might be the problem and how can I fix it?

I’ll guess that your theme is specifying a too-small size for that image. Can’t help more without a link to a functional example or at least the name of the theme.

It’s a stock theme, haven’t added any other tbh.
Link to the site is

That looks like Source? Since you haven’t made changes to it, can I get you to reload it? (Settings > design and navigation, and then choose it again.) I’m hoping that since Source is new and getting active bug fixes, that grabbing the latest version will fix the issue. :)