I'm not an expert either, but I think you could also use a one time pad to do exactly that.

You have a picture of a cat, and 2 one time pads (OTPs). OTP #1 is the key for your real data, and you can generate OTP #2 such that it decrypts the ciphertext (in this case, an image) into whatever data you pick.

Whether this is practical is a completely different question though.