Wait, do you have something to see a decoded image? If yes, just introduce modifications. Flip a bit or increment some octet and see what happens.
As for the theoretical part: with zero knowledge about the content of the message and the encoding mechanism, you can never decode it. That is because you have no way of discerning valid decodings. Which is why one-time-pad is provably unbreakable encryption even if you could bruteforce all possible keys. Same goes for single blocks of many block ciphers.
IMO communicating with extraterrestial species would be much worse than just trouble with encoding. If both sides had the same goals while designing the communication methods, both would probably end up with something recognizable to the other side. The true obstacle may be of different nature. No concepts shared. Humans love to model ETs as “just an exotic civilization”, but in reality they may have been shaped to use concepts completely unfamiliar to humans. Make a simple thought experiment that doesn’t even require involving other planets. How would you talk with ants or trees, given you had a chance to do so? You may argue they are not complex enough and sure I do not ask about having a debate on philosophy with a nearby ant hill. But if you consider them horribly dense and only want to communicate simple things… you still can’t. Their and your pereception of everything is just too different to share any kind of information.