* 18:00:25  WARN <GdkPixbuf> Cannot open pixbuf loader module file '/Users/john/Development/Gnucash-Build/Gnucash-2.6-Leopard/inst/lib/gdk-pixbuf-2.0/2.10.0/loaders.cache': No such file or directory

This likely means that your installation is broken.
Try running the command
  gdk-pixbuf-query-loaders > /Users/john/Development/Gnucash-Build/Gnucash-2.6-Leopard/inst/lib/gdk-pixbuf-2.0/2.10.0/loaders.cache
to make things work again for the time being.
* 18:00:25  CRIT <gnc.gui> [gnc_gnome_get_gdkpixbuf()] Could not load pixbuf: Couldn't recognize the image file format for file '/Applications/Gnucash.app/Contents/Resources/share/gnucash/pixmaps/gnucash-icon-16x16.png'
* 18:00:25  WARN <gnc.gui> error loading image from [/Applications/Gnucash.app/Contents/Resources/share/gnucash/pixmaps/gnucash-icon-16x16.png]
* 18:00:25  CRIT <gnc.gui> [gnc_gnome_get_gdkpixbuf()] Could not load pixbuf: Couldn't recognize the image file format for file '/Applications/Gnucash.app/Contents/Resources/share/gnucash/pixmaps/gnucash-icon-32x32.png'
* 18:00:25  WARN <gnc.gui> error loading image from [/Applications/Gnucash.app/Contents/Resources/share/gnucash/pixmaps/gnucash-icon-32x32.png]
* 18:00:25  CRIT <gnc.gui> [gnc_gnome_get_gdkpixbuf()] Could not load pixbuf: Couldn't recognize the image file format for file '/Applications/Gnucash.app/Contents/Resources/share/gnucash/pixmaps/gnucash-icon-48x48.png'
* 18:00:25  WARN <gnc.gui> error loading image from [/Applications/Gnucash.app/Contents/Resources/share/gnucash/pixmaps/gnucash-icon-48x48.png]
* 18:00:25 OTHER <GLib-GIO> No GSettings schemas are installed on the system