@genegold In these particular cases, use the heart icon to set the thumbnail that you want for a speed dial and then restart Opera to see the change. Opera has some quirkyness with imported speed dials or speed dials that are treated as imported because a "Bookmarks" file was used just that came from a different Opera.
If all else fails, delete the speed dial, empty the bookmarks trash and the URL opera://bookmarks, goto the site and click the heart icon to add the site back in.