You might consider using photos that have more than the target object in them. It is easy as heck for a computer to determine the bounds of an object in a photo

You might consider using photos that have more than the target object in them. It is easy as heck for a computer to determine the bounds of an object in a photo, and increasingly easy for it to know what that object is. It's certainly no challenge to a bot to determine the bounds of the "coffeemug" when it is the only object in the photo. Also, CAPTCHA of any sort just sucks. As a user, I only will bother with a site that uses CAPTCHA if it is providing something I absolutely must have. If it's something I may or may not use, and I'm presented with a CAPTCHA (of any kind), I leave.