I am sure you have heard flickr being widely used for uploading images. The fact that it offers substantial capacity for upload (and more so for professional accounts) makes it all the more handy. I have long stopped storing photos locally. I just upload them and delete them off my hard-drive.
However, here's a thought-provoking article on how images stored on flickr can be used to store hidden information. The technique is called steganography. In simple terms it is the science of hiding messages in such a way that only the person looking for it knows its there. An innocent looking image file, for example, could very well be hiding secret text messages. And in case of services like flickr where they are millions of images being uploaded non-stop, this might easily go unnoticed. Keith explores this possibility in an article on his blog titled "Steganography with flickr".