I can't replace WinRAR file in Minecraft Resource pack, why?

I downloaded a resource pack from the Internet and then copied the icon file from the folder to my desktop and then edited it with paint. When I was done I wanted to move it into the folder to apply the new icons, but then the error message came up (see below).
I hope someone can help me

LG

Til

You have probably opened the archive file one more time.

I had Minecraft open so it didn't work

Yes