@kaizu I see you already have it solved, but, to answer your question:
The "Bookmarks" file in the profile folder is a JSON file. You can edit it with a text editor or with JSONedit (which is nicer). Since the speed dial entries are just bookmarks in a bookmarks "speed dial" folder, you'll see the start page entries right there in the "Bookmarks" file.
Now, the speed dial entries are special though in that they have an imageID that's references a thumbnail. That imageID and its thumbnanil (as base64 data) are stored in the "BookmarksExtras" for each speed dial. The "BookmarksExtras" file is also a JSON file that you can edit.
In short, use something like https://www.base64-image.de/ to base64-encode the image you want to use for a thumbnail, find the imageID in "BookmarksExtras" for the speed dial you want to modify, and replace the existing base64 data with the data for the new image (all while Opera is closed). Then, you can customize things how you like.
Now, there is another way to do things. Directly load the URL to an image you want as a thumbnail and bookmark that image to the speed dial folder. Then, close Opera and edit the "Bookmarks" file. There, find the entry for the image, change its URL to the site/page you want, and change its name to what you want. In this case, you won't have to mess with the data in "BookmarksExtras" because the data for the image you bookmarked will already be there.