In the past, I changed my folder icons for Applications, Downloads, Documents and for applications like Mail Safari. Now many have reverted or gone to a white page picture.
How can I replace them with my own icons again?
The manual way is to go to the system folder using finder go to folder command shortcut Shift Command G ⇧⌘G
Rename the original icon and then name the new icon file with the original name. Will need to be an admin user and enter your password when doing this
Using a dedicated program may be easier see Liteicon for a free download.
The file will need to be a Mac icon file .icns