Big Sur Red Box Images in Preview

Rifles wrote:

When clicking on an image to view an image, just before image is shown a red oblong box appears then goes away!

Uninstall all third party apps that are Cleaners/Optimizers/Anti-Virus

To trouble shoot further you can:

—Try a SafeBoot https://support.apple.com/en-us/HT201262

Takes noticeable longer to get to the login screen, does a 5-15 minute disk repair before it fully boots up, and certain system caches get cleared and rebuilt, including dynamic loader cache, etc. Login and test. Reboot as normal and test as caches get rebuilt.

In Safe mode third party system modifications and system accelerations are disabled, it removes malware, etc hampering smooth operation, however a reboot will put it back to normal mode.

This test will tell you if third party interference; extensions etc are not loaded in safe boot mode.

—Test issue in another user (or guest user) account  https://support.apple.com/en-us/HT204443

This will tell you if it a universal issue or isolated to your user/admin account. 

If no resolve you can file a bug report—Apple Feedback http://www.apple.com/feedback