I cannot seem to find a solution to bring back the original icons and thumbnails. All I did was upgrade unity and right out of the box this happened.
Try to go to the Project folder.
If you are on Windows: In the Windows Explore
On the Mac: In Finder.
Then go to the Assets Folder ad find the images.
Maybe there you can find the original images.
If yes.Copy them to somewhere else and delate them in Unity.
Then add them again.
I hope I helped you
Found out I had to click re-import on the assets folder inside Unity. Happened after reverting back from the lightweight rendering pipeline.