The problem

The wrong thumbnail is chosen for blog posts on minecraft.net - rather than using the large header image, it picks another image from within the page - usually the author's avatar (which doesn't work too well). This did not happen with mojang.com blog posts, although it seems that that was only the case because there were few other images.

You can see this by comparing the domain listing for minecraft.net (archive) with the domain listing for mojang.com (archive).

Cause

This happens because minecraft.net does not specify a thumbnail, so Reddit instead picks the thumbnail automatically. Reddit prioritizes large images that are mostly square for thumbnails; the images on Minecraft.net don't fit under that category. You can see the code that reddit uses to scrape thumbnails here.

Fix

This can be fixed either by adding a <link rel="image_src" href="..."> tag or a <meta property="og:image" content="..."> tag. The later is part of the open graph protocol while the former is a special link tag used by Reddit and certain other sites. I personally would recommend using the OG tag but either works.