They seem to be shown at roughly 460x306, so there's no upscaling happening.
We are both right! The problem only becomes apparent when looking at the page on a medium or small sized window. Below 1028px wide and the preview image drops below the description, and expands to fill the window. The responsive design appears not to have a max-width of 100%.
Ah, yes! It pops up to about 960x667 on a narrower display. Minimum laptop dimensions are 1024x768, which seems to trigger it. The breakpoint should probably be set to 1023px to avoid that, which would take care of laptops, as well as iPads held horizontally.
True. The break point it too wide. It should probably be at around 940 where you also get the hamburger menu instead of the regular one. I'll check it...
It will still be blurry between 500 and 940px though, since we have a fixed rescaling for all the screenshots to a width of 480px.
I doubt that anyone is spending a lot of time gazing lovingly at the skin previews, especially on tablets or phones. As long as ordinary laptops don't trigger the oversized thumbnails, that's probably good enough.