Looks like google limits search query length to 2048 characters. That’s probably enough room to exclude a majority of the biggest names.

It would be nice if there were a way to make the exclusion list de the default for all your queries. For instance, I never want to see results from WikiHow again. Ever. Or the New York Times or any of the other paywalled sites...

unpinterested is an extension which simply adds -site:pinterest to image searches. I don't think it'd be hard to do something similar with a custom list.