How could I trace the source of 'gtk-missing-image' warning message? The complete warning looks like this: Gtk: warning: Could not find the icon 'gtk-missing-image'. The 'hicolor' theme was not found either, perhaps you need to install it. You can get a copy from: http://icon-theme.freedesktop.org/releases
This is the Claws Mail bug tracker, and not a help forum. Plus you are asking about GTK and not about Claws Mail.