Do an F5 on the SpeedDial site, it should refresh the thumbnails.
Copying the Bookmarks file should be enough.
For an easy recovery, apart of periodicall back ups, you can:Create a profile with your desired settings without bookmarks or sync enabled and save it. Other, adding the extensions
Then do periodical back ups of the Bookmarks and Extensions settings.
At any problem, you just need to use the "Advanced profile" Settings+Extensions and import your Bookmarks and Extensions settings, in case that the previous back up is damaged or if you want to use more profiles.