It has been demonstrated that files can survive flickr. More precisely, a hidden file in a flickr image survives being uploaded and then downloaded again. This means that flickr can be used as a covert means of message transmission. Not a massive deal, given that images with hidden files can just as easily be sent by many other means, but it is interesting given flickr's massive storage capability. Short of manipulating the original file to an intolerable degree and thus defeating the purpose of flickr, I don't see that there is anything they can do about this.
Why an intolerable degree? If the original file is small, then it can be put into the image with a lot of redundancy and thus reconstructed, this is the same idea that is used to embed a digital watermark in an image which can survive photoshopping.
Aside: Yes, I'm referring to an old post, but I just remembered that I hadn't linked to it when I read it for the first time, and so I've just googled for it again!