I can reproduce the same behaviour: when launching a second instance of gthumb with a command line argument (being an existing image), gthumb does not open the file (neither in a new instance nor in the opened one), with the error message
GDBus.Error:org.freedesktop.DBus.Error.NotSupported: Application does not handle command line arguments >gthumb --version gthumb 3.3.1, Copyright © 2001-2010 Free Software Foundation, Inc. aptitude version: Version: 3:3.3.1-2+b2 running : Linux 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt2-1 (2014-12-08) x86_64 GNU/Linux Best, Ara
