I've done a little more research on this
I was intrigued...
My site uses a custom blog, not a public platform, if you visit it (the blog area) it looks alike wordpress but it isn't. We also use NetDNA to serve images, which is the parent company of maxcdn.
My image search in Google returns to my posts, not the images (but there are 2 differences with your site: custom URLs on NetDNA (also possible with maxcdn) and not a Wordpress site).
Then I checked for a small blog I write about SEO, works with wordpress but without NetDNA (CloudFlare instead as the CDN) and the images in google search also link to my posts.
So that leaves with only one option, custom domains. You are serving images using a third party domain, which can be seen as a hotlink by Google and therefore they may link to the domain hosting the image, as it is probably the image owner.
This are just guesses, but it looks like a nice/possible theory.
Why don't you start using custom domains with maxcdn and see what happens?