Notice referrers from image searches, like Google Image Search?
Continue to optimize the images you upload to your posts: use clear, succinct file names and add alternative text and captions, which you can do with each image via the option in your Media Library.
You can also simplify Google Analytics by getting the Become A Blogger Dashboard.
Within Google Analytics you can measure the amount of traffic coming to your blog from most social networks including Pinterest.
Here, you can access your referrers for the current day and yesterday.