Return default placeholder image when image file on server not found

[JoakimR]

Found the answer on IRC #Freenode #nginx by catbeard

location ~ \.(png|jp?g|gif)$ {
        	error_page 404 /404.png;
       	}

http://serverfault.com/a/481612

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,267211,267213#msg-267213