Currently there's no way to "add" a picture in the clipboard (copied from a browser) to the track via track info panel.
Among the several context menu items, "Save Picture to" command always duplicates the currently displayed picture on the track info panel.
The second one "Paste" always replaces the existing picture. For instance, if a file has an embedded image pasting a new image as folder.jpg removes the embedded image. Likewise when it has an external image linked, pasting a new image as an embedded image removes the external image.
Lastly "Choose Picture" command only works when you have an image file in the same folder with the track.
I think Paste command should "add" a picture rather than "replace" an existing one (except when it explicitly mentions "original picture"). Most people wouldn't expect running "Paste" command with a new image would delete an existing embedded or linked image without confirmation.
Deleting any images should only be done via a separate command. In fact it would be great if track info panel could also delete a displayed picture via context menu. Currently only tag editor and artwork manager are able to delete a picture.