Photos won't open or preview after copying

[fiike]

Hi! I copied photos in a new folder from the same computer and suddenly they're not working properly anymore.
- for those files that still have the thumbnail, some would show the message (the one in the screenshot) when I double click. I can still open it with a right click but this message pop up:
"macOS cannot verify the developer of “1570103165271.jpg”. Are you sure you want to open it?"
- some files just lost the preview, as you can see in the screenshot they have just gone blank. I can still open them with a preview but not by double click. Also, I cannot upload those files online, they are not being recognised.
Does anyone know how to fix this? I'm not a very technical..

 

[Cory Cooper]

Hello and welcome.

I have never seen this issue with anything other than applications.

-Which version of OS X/macOS?
-Where were the original photos - in a folder, iPhoto/Photos, or did you copy them in Finder from All My Files?

C

 

[fiike]

It is MacOS Catalina 10.15.4, the original files were in a different folder on desktop and worked fine before. I tried restarting the computer but nothing changed..