I generated a patch which had a new image file new it along with some text changes to an existing file. Applying this patch again gave me:

sindhu@arch ~/code/gnome/zenity/help/C (git)-[master] % git am ~/tmp/patches/Add-user-help-for---listen-bug-711396.patch   
Applying: Add user help for --listen, bug 711396
error: cannot apply binary patch to 'help/C/figures/zenity-notification-listen-screenshot.png' without full index line
error: help/C/figures/zenity-notification-listen-screenshot.png: patch does not apply
Patch failed at 0001 Add user help for --listen, bug 711396
The copy of the patch that failed is found in:
   /home/sindhu/code/gnome/zenity/.git/rebase-apply/patch
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

The line of interest here is: 

error: cannot apply binary patch to 'help/C/figures/zenity-notification-listen-screenshot.png' without full index line

I generated patch for same commit with `git am` and compared contents of both these patches (git am- generated and gitg-generated) and found that `git am` patch had the binary changes in full whereas gitg-generated patch did not.

`git am` generated patch had http://fpaste.org/57629/13857106/
gitg generated patch had http://fpaste.org/57630/71065513/