Until recently, you were able to trick Opera into adding an image as preview for a different page, since not all the pages had an image that would fit the preview. You could bookmark the image and then change the URL to the desired one, now is not possible and it was a feature that worked quite well for bookmarks of different content on the same page, like YouTube or streaming video services. I suggest you add the feature to add custom bookmarks previews and store them even when the URL is modified.