$ git diff
--- a/anImageThatHasChanged.jpg
+++ b/anImageThatHasChanged.jpg
@@ -1,5 +1,5 @@
 ExifTool Version Number         : 9.76
-File Size                       : 133 kB
+File Size                       : 54 kB
 File Access Date/Time           : 2015:05:02 20:01:21-07:00
 File Type                       : JPEG
 MIME Type                       : image/jpeg

$ git diff-image
# The same output as above, *and* a montage of the visual differences will be
# generated and opened in Preview.

Installation

Install exiftool and ImageMagick. (The script will cope with these missing, but it's not going to be very exciting without them.)

brew install exiftool imagemagick

Run the install script, which will configure your global git config for you. It will tell you what it's done, so it should look something like this:

~/git-diff-image $ ./install.sh
+ git config --global core.attributesfile '~/.gitattributes'
+ touch '/Users/yourname/.gitattributes'
+ echo '*.gif diff=image' >>'/Users/yourname/.gitattributes'
+ echo '*.jpeg diff=image' >>'/Users/yourname/.gitattributes'
+ echo '*.jpg diff=image' >>'/Users/yourname/.gitattributes'
+ echo '*.png diff=image' >>'/Users/yourname/.gitattributes'
+ git config --global alias.diff-image '!f() { GIT_DIFF_IMAGE_ENABLED=1 git diff "$@"; }; f'
+ git config --global diff.image.command '~/git-diff-image/git_diff_image'

Public domain dedication

The files in this repository are by Ewan Mellor, and are dedicated to the public domain. To the extent possible under law, Ewan Mellor has waived all copyright and related or neighboring rights to this work. http://creativecommons.org/publicdomain/zero/1.0/.