while waiting official bookmarks manager from Opera devs (if ever comes), there is a nice user extension (still in development, tho, but fully functional). You need to install it through developer mode. It’s called Real Bookmarks, and I’m using it.
http://my.opera.com/Sumpfkrautjunkie/blog/2013/07/16/export-tooltips-thumnails-thumbnail-mode
…link to zip file is at the end of article