I recently upgraded to macOS Monterey (yeah, finally) and immediately noticed that previews of new images weren’t showing up in Finder.
Old images did have a preview image so something was wrong with how previews were generated.
Searching online, I found a few solutions, including one that involved deleting the Finder configuration file (plist file) which may have worked in 2009 or 2012 but that was long ago and it doesn’t work now.
So, to save you doing that and having to resize your favourite Finder windows and configure settings again, the answer is very simple and I found it here.
Activity Monitor and search for
com.apple.quicklook.ThumbnailsAgent. Double click on it and force it to quit.
CMD-right click on Finder in the dock and relaunch Finder and your thumbnails should now appear in all their tiny glory.
I’m writing this here because apparently this happens to people over and over again. While it’s the first time I’ve seen this problem it probably won’t be the last.