Photos library

The Photos Library should not be in Applications, only the Photos.app.

If you really managed to drag the Photos Library to Applications or to create a library there, you cannot simply drag it back. The Applications folder is protected, and dragging items from Applications will just create an Alias. If that happens, hold down the options key ⌥ while dragging the Photos Library into the Pictures folder. This will create a copy. Then delete the original from Applications.

But yes, if the library (called photos library.photoslibrary - and not the application) is in the application folder you can move it just by dragging it to the pictures folder.

You say you've tried that without success - how did you try, and what happened when you did.

Are you sure it is the library, or just the application?

Here is the icon for the application: