How to Delete Photos from Mac Desktop without Actually Deleting them from iPhoto Library?

The desktop picture is controlled in the "System Preferences" pane on your Mac. If you want to delete photos from Mac desktop without actually deleting them from iPhoto Library, then open "System Preferences" on your Mac. Click "Desktop and Screen Saver" button and then click "Desktop" tab and then drag the new photo to the image of the desktop.

You can create a new folder for your desktop pictures. To create a new folder, click "+" sign located in the left pane and select a new folder location and copy your desktop picture into that folder using the Finder. You will see that folder when you open Desktop and Screen Saver. When you open the folder, you will see all the photos that it contains, click on the one you want.

If you are using multiple screens, then you can drag the Desktop and Screen Saver window to another desktop. You can also change its photos. You don't need to close and reopen Desktop and Screen Saver pane each time.