Update: 07/05/2010, as of today, snapshot builds of 10.54 have been tweaked, and rendering of thumbnails has previously been much improved. Unfortunately you may have to redo your thumbnails, making sure they are 256×192 (to stop them being stretched). Incidentally, this is how my speed dial currently looks.
The thumbnails I used can be downloaded here (.zip) Updated for 10.54, and the background image can be found here. I'm happy enough with how it's looking
If you want to do this yourself, you'll need Opera 10. To have a transparent thumbnail background you will need a skin with one (for example Speed Dial - Clear which I'm using). Then follow Tamil's tutorial on how to change speed dial thumbnails. The Opera Image Dial Generator is a great tool to generate your own thumbnails.
Thanks MyOpera for a great contest! Looking forward to the next one as always!