access-protection

I would like to create a web site with many images. But I would like to protect against direct access to images, e.g. direct links to images without visiting the web site. What is the preferred way to do this? And what are the alternatives with Pros and cons? I have some ideas (I don't know if they are possible): File permissions PHP Sessions Temporary file names or URLs HTTP Redirection? Maybe this isn't practiced on many web sites? E.g. I tried to access a private photo on Facebook without beeing logged in, but I could still visit the photo. The platform will probably be a Ubuntu machine

Apparently this key-oriented access-protection pattern : class SomeKey { friend class Foo; SomeKey() {} // possibly non-copyable too }; class Bar { public: void protectedMethod(SomeKey); // only friends of SomeKey have access }; ... doesn't have a known name yet, thus i'd like to find a good one for it so we can refer to it without breaking our tongues. Suggestions? It should be: succinct convey the intent of access-protection ideally imply that no proxying is required (?) I like, in decreasing preference: passkey friend idiom passkey-door friend idiom pass-door friend idiom key-door friend