When you save a site using the Opera Speed Dial feature, it uses an image of the site's web page as the image for the speed dial button. Most of the times the image is not recognizable since the image is so small. If you want to, Opera lets you customize the logo used on the speed dial buttons.

To customize the speed dial image, first get yourself an image to use that is in the .PNG format. My personal preference is to use the site's logo as the image on the speed dial button. But do whatever works for you.

You then need to find the thumbnails directory in your Opera profile (opera:config).

You can then replace the image in the thumbnails directory with your new image. The key is to then change the file attributes to be read-only (the method for doing this is system-dependent) so that Opera can not update the image.