Images being sold/stolen

I believe that if you post a photo on the internet without copywiting it, it is for anyone's taking or use. Images become free for anyone's use when it goes on the internet, just like anything else on the internet. If you have not copywrited it, then why cant someone else use it? The entire point of puting something on the internet is to share it, and not with just select people, but the entire world. If you put an image on the internet without a copywrite, but dont want it to be used in any way others would like to use it, then thats a big mistake on your part. The internet is free reign, and is only restricted by legal agreements, whether its a copywrite, or a user agreement with facebook regarding your images. The only difference between puting an image on the internet and puting it on a giant blimp to fly through the sky is that its easyer to obtain that image on the internet.