Cool trick, I have encountered something like this in a steganography wargame before, the only difference is they used Base64 encoding on the original picture instead of AES :)
In the general usage case base64 is not even obfuscation, really (though it does make it non-human-readable... for most humans), just an encoding for dealing with situations where you need to store or transmit 8-bit or multibyte character data into a system that may otherwise be incapable of dealing with it reliably.
Of course it's not, but the method is the same, you append a different picture to the end and modify the original image header to point to it. The encoding or encryption is not relevant.