Are you sure they aren't there? I think I recall seeing them while I was testing this procedure.
I edited thumbnails.db exactly on the way like you described in the linked post. There are no entries in thumbnails.db of google.com, youtube.com, wikipedia.org and a few other most used sites ...
Are you saying that if you create a new SD for google.com, say, the new thumbnail doesn't have an image of the Google search page?
Exactly, it doesn't take a screenshot of the google page. instead it is using the logo of google. Thats why i am writing here, to make opera use another image for google.com's SD.
By the way, i am using Opera v. 22.0.1471.70