A: Query this search string with your favorite search engine:
"Image Source * via * on Pinterest" - over half a million images embedded already.
Q: How many images are both misattributed to Google, and embedded in a 4th party website?
A: Query "Image Source: google * via * on Pinterest" - at least 30,000.
Q: How many are embedding images from your website?
A: Query "Image Source: mywebsite.com * via * on Pinterest"
These tips WILL NOT help you find the more literate webmasters that strip the code down to straight-up hotlinking.
Slowly, but surely, image copyright will be pinned to oblivion.