First you have to disable thumbnail refreshment, to do that, open Terminal in /Applications/Utilities/ and run:
defaults write com.apple.Safari DebugSnapshotsUpdatePolicy -int 2
Then, go to ~/Library/Caches/com.apple.Safari/Webpage Previews.
If the user Library folder is hidden, you can access it by pressing Command + Shift + G (⌘⌃G) in Finder, pasting the path in the text field and clicking “Go”.
In that folder you will find jpeg files with 32 character names. Either edit those directly, or copy images to the folder and rename them to whatever the name the different websites’ thumbnails had.
Remember to keep a backup of the files, since they are stored in a cache folder which might be emptied by certain programs or upon installing a new system.