[inzzzomnia]

Not sure but I think the new scraper that's in the making will let you use other sources so hopefully it will soon be a non issue.

True. Sadly IMDb is also stuck at 400x225px and TMDb doesn't have thumbs for everything.

The images at IMDb are usually available at a higher resolution if you tweak the URL but they aren't that consistent with thumbs.
TMDb is by far the best with TVmaze as a good complement.
I usually get them from each shows homepage so I get the "offical" thumb, but I'm kind of picky