1. You must hack your phone. If you haven't yet, hack it right now!
2. Download X-Plorer (Free). Install it to your phone.
3. Open X-Plorer. Go to: C: | E:/System/apps/Opera Mini/ds . Open 'ds' folder. You'll see various files! Some of these are speed dial images!
4. Speed dial images are around 4.15 kb. Click on them. If they open in image viewer, they are your desire speed dial thumbnails . Copy the file name of a speed dial image which you want to change.
5. Select an image which you want to replace as the speed dial background. Resize it into: 113 × 113 . And rename it as previous one. [ex: if the name of the speed dial image is file41032, then rename new image as file41032]
6. Move the new renamed image to C: | E:/System/apps/Opera Mini/ds . Now open Opera Mini and it's done!
See, I made mine: