Let users paste the image itself by pressing Ctrl+V/Cmd+V here:

Users don't really save an image to disk when they just want to search for something on their screen. They just capture it to the clipboard. Such a feature would prevent users from going through the hassle of pasting the image to some other editor, saving it to disk, and try to find it on the disk again when uploading it. Google Image Search already supports this.
It would be useful on mobile too as copying screenshots to clipboard is also common there.