Sankaku Channel/

ALAKTORN said:
^^Maybe they weren’t downloaded properly? Try redownloading them.

That was of course the first thing I tried but it doesn't work.
p.s. In case browser is related to the issue: I use Firefox.

System said:
Thumbnail issues on your computer should be nothing to do with the site - the original images are entirely untouched.

Aren't the preview versions generated by the site?