I have tried to find an answer to this question, but have been unsuccessful. However, I did find a website that explains step-by-step how to do it for an older version of Opera.
How can you change the speed dial thumbnails for Opera 22.0, or the latest version of Opera currently?
Go to http://www.symbaloo.com and create your own set of thumbnails.
Install an Opera extension called "Custom Home Page" or "Homepage in New Tab" or "New Start Page Pro", and copy the address of your Symbaloo mix into the extension.
Start enjoying your new customizable Opera speed dial.