I too would like to be able to set the speed-dial button icons to pictures of my own choosing.
The steps in the 3rd post down (by rafaelluik) no longer work (as of May 19, 2015). The thumbs.db file is still there, but it does not reflect what speed-dials are active. It seems that when I add or change a speed-dial is is only the BOOKMARKS file that gets updated. I'm guessing that as the developers are integrating Speed-Dial with Bookmarks...all data related to the bookmarks & dials is going in to the BOOKMARKS file.
The suggestion to use SYMBALOO does not solve the issue. Symbaloo is a third-party site that is not part of Opera. There are many bookmark-managers/speed-dial type sites on the Internet. Using those is okay, if that is what you want. But suggesting that as a solution to this problem is like saying: If your car is broken...don't take it to the mechanic, buy a different car. It's an option, but not a solution to the problem.
Saying that we can choose which picture using the Bookmarks / Heart-Menu is not a solution either. It's nice that they put up a couple of pictures for us to choose -- but in many cases I (and many other comments I have read around the web) want the ability to SET OUR OWN picture.
I have confidence the developers are working on this as they integrate speed-dial with bookmarks. I like Opera and will wait patiently for this feature.