GitX is available at https://github.com/brotherbard/gitx/downloads (Get the experimental branch)

Git Tower is available at http://www.git-tower.com/

Textmate has a GIT Bundle at https://github.com/jcf/git-tmbundle (It's quite good, but maybe not for designers).

And Giggle (http://live.gnome.org/giggle).

Windows is the only OS lacking a good client, but I use GitX for Mac every day, as do the less technical "devs" at our Startup.

Since the target group were designers, the presenter used a Mac, and tried to do everything using a GUI client for Git. I don't remember which one he used, but it ran natively on the Mac and it looked quite nice.

(Still, the presenter sometimes struggled with that GUI. He was obviously more used to the command line.)

Simple pixel-by-pixel "diffs" are of course possible, but only useful in trivial cases.

What if someone changed the color scheme (which affects almost all pixels)? What if someone moved some part of the image to another part? What about resolution/size changes and multiple layers? What about vector graphics?

And the most important thing: How to display that in a way to be easily understood by non-technical people?

For hard-core techies, there is of course the option to use a text-based image format. For raster graphics, the "plain" variants of PNM come to mind. For vector graphics, SVG or EPS might be good choices. Then, a good (indention-aware) textual diff should produce sensible results - especially if only details were changed.

"merge"

Automatic merges are only useful if they aren't too "clever". That's important for text and especially important for graphics.

So if two distinct areas of an image are edited, a simple merge can and will work. But overlapping changes or even global changes should always result in a conflict.

However, if only trivial merges are desirable, most changes will cause conflicts, which would be not much different from the current situation.

Also, that kind of merges will already happen automatically with the current (text) merge anyway, provided that formats like PNM, SVG and EPS are used.

http://www.kaleidoscopeapp.com/

That's what the commit comments are for.

BTW, programmers also prefer reading commit comments, commit dates and author names. They don't read hashes. (Although they sometimes copy&paste them because they are short and unique.)

On top of this, Git added the ability to merge changes together automatically. Imagine that one person changed a bunch of stuff in the first paragraph in a document, and someone else changed a bunch of stuff in the second paragraph, and the computer could automatically merge those changes into one document.