The good news: you can actually use these images (a link back would be appreciated).
The bad news: these are free to use ONLY after you have bought the reference images from Shutterstock. Using them without a licence will constitute copyright infringement. The price is very moderate: 29 dollars for 5 small images (or 2 large jpegs or vectors); that won’t break the bank.
To buy the reference image click here. Enjoy!