There was a great white board Friday back in May last year in this, take a look at this :
When a third of all searches performed in Google are for images and 12.5% of SERPs show Image Pack results, you know it's not a facet of
SEO to be trifled with. Today's episode of Whiteboard Friday is densely packed with all the image SEO tips you could ever want, from the bare basics to ranking factors to important next-steps.
Image SEO basics:
Look, there are a couple of things that we should discuss before we even get into that. First off, many folks are asking...
1. "Is it still even worthwhile to do Image Search SEO?"
You might recall that in years past, when you clicked on an image in Google Image Search, you would go to the website or the webpage that had that image on it. Now, if you click it, Google opens up a full-sized version of that, tells you the website it's on. But for many people, that's enough. They do a bunch of searches and never actually visit your website. Google is just serving up your image unbeknownst to you, and you don't even get any data on that. So you don't know how many impressions it gets, all that kind of stuff. So here's the case for Image Search SEO.
Read more....
https://moz.com/blog/seo-photos-visuals-graphics-whiteboard-friday