Valid Strategy For Secure Image Upload From Flash Client?

I'm creating a flash application that will post images to a url for saving to disk/display later. I was wondering what are some suggested strategies for making this secure enough so that the upload is verified as coming from the application and not just some random form post. Is it reliable enough to check referring location realizing that I don't need bulletproof security, or perhaps setting authentication headers is a better strategy even though it seems unreliable from what I have read. The application and the server script will reside on the same domain and is in java - is there a way to check for a 'session' or something like that?

Another thought I had was some sort of simple hashed key type system, I could hard-code a key into the flash application and pass something to the server based on that - the server would also know this key and be able to verify if the value passed was based on that? The app is a public app, so authenticating users is not an option. After more research I am thinking about using a hard coded salt key on both ends, then sending an MD5 hash of the base64 encoded image bytearray+salt to be matched on the server side.