How do differentiate if the image is access via <img src='img.jpg'> and http://somesite.com/img.jpg (directly in the url?)

If the img is directly accessed in the url of web browser, i want that to be redirected to website home page or some other page where the image belongs.

http://i683.photobucket.com/albums/vv191/angelinesona/Animhorse.gif

If i enter this image url in browser url, it will go to some html page and displays this image. I want this to be done in with the help of nginx.
Please kindly advice.