I feel like this is some really compelling tech. It would be so amazing to build stuff with this in mind. I wouldn't be comfortable doing it, though. This sort of API is available only until Google decide that they don't want it to be available. There's not really anything close to equivalent that you could drop in to replace it if it were being shut down, the price were being hiked, or you had some sort of other iss…
curl -H "Authorization: Bearer {access_token}" --data-urlencode "url=YOUR_IMAGE_URL.jpg" "https://api.clarifai.com/v1/tag/"
...you get the top 20 tags for that image and their confidence levels, too. Their homepage at Clarifai.com has a pretty good demo that lets you see it in a more visual way.