How to measure traffic from Google Images
This is a quite simple way to measure effectively traffic from Google Images search engine. Unfortunately it does not work for images appearing at universal search but worth it.
You might argue that traffic from images is low. Take a look at this old article where I found some sites having up to 12% of the total traffic from images, not bad eh?
Obviously some webs rely more of this type of media that others but even when the percentage is low, the data you get is interesting because it helps a lot to understand how the work you did to SEO optimize your images for search engines is performing.
How to
Just add this simple line to your Google Analytics tracking code
_gaq.push(['_addOrganic', 'google', 'q', true, 'google images', '/imgres?']);
something like
<script type="text/javascript">
var _gaq = _gaq || []; _gaq.push(['_setAccount', 'UA-0000000-1']);
_gaq.push(['_addOrganic', 'google', 'q', true, 'google images', '/imgres?']);
...
Here you will find the parameters of _addOrganic() explained. Well, they only explain 3 parameters but that implementation adds five. Why they are not described? Who knows, maybe at Google they thought they are self-explanatory.
What do I get from that?
First of all you get that traffic labeled properly as source 'google images' what lets you more accuracy in the numbers as you are now able to segment it.
Secondly you get the keywords users queried the engine. Apparently there aren't (not provided) here so you get 100% of them which is fabulous.

You also get the landing pages where visitor headed after clicking at one of your images in the SERPs.

Exporting data directly from Google Analytics to Excel and a bit of the magic that pivot tables add, insights pop up right away.

Of course there you have the rest of metrics you usually get like Visits, Revenue, Conversions, Pages / Visit, Avg. Visit Duration, Bounce Rate, and such. Did you say Revenue? Yes, take a look.

It won't make you reach but it pays for some good diner. There are sectors where having good images is important.

Traffic from images might drive some revenue but it surely helps assisting conversions as you can see below if you implement that simple line onto your tracking code.

The more you segment the more you know.
OK but what about the image bringing traffic?
One of the parameters coming in the referral string is the exact path to the image matching the query:
imgurl=http://dynamical.biz/blog/image/ani-lopez-data-analyst.jpg
With a bit of patience and a filter you could capture that extra info in a User Defined Value but I'll let you deal with that if you don't mind.
Enjoy it!