TonyStill, that is the problem!
The law is essentially guilty until proven innocent. (This is why it isn't a good law.) The file may not even be encrypted - it may be random garbage, such as you might get, say, after a disk recovery.
They can pick a file and demand that you decrypt it - if you can't, you are guilty, unless you can prove that it doesn't contain encrypted data. Good luck with that.
Regarding 'undetectability' - OK, some may be detectable. If you have two pictures, one with and one without your message, that is a giveaway, for example. If you've ever edited a JPEG in some non-obvious manner they might pick this up as a false-positive.
IIRC a Register article described an analysis which suggested that 25% of pictures on the internet may contain steganographic data. Doesn't reassure me.