The method I have successfully used to display thumbnails that I have collected via screen capture or from online image sites is as follows

1. Save the desired website in speed dial
2. Obtain or create the new image that you would like displayed in Speedial and save ot as a 200x133(aprox) PNG file.
   3.In C:\Documents and Settings\USER 1\Local Settings\Application Data\Opera\Opera\thumbnails locate the existing thumbnail and select rename. Highlight and copy the name
3. Select the new PNG image, select Rename and paste the name from Opera's Speed dial image
4. Delete the existing image from C:\Documents and Settings\USER 1\Local Settings\Application Data\Opera\Opera\thumbnails
5. Paste the new renamed image in Opera's thumbnail file
6. In Opera:config select User Preferences and goto Speeddial Thumbnail Request Header and de-select the checkbox

It seems to me that Opera V.22 has many bookmarking options. They do not emulate the Bookmark menu with which you are familiar but the functionality is still there. Between that bookmarks bar, speed dial and stash I have found there is no lack of bookmark saving options.
I agree with you that at first glance it seems as Opera has been left wanting but I have found that with time I am able to navigate to my favourite pages with ease.
I have further found that in reality there are fewer than 10 or so bookmarks that I access with any kind of regularity.