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More importantly, all encryption keys are either fixed or stored directly alongside ciphertext.

The cryptography in this system seems to be 100% cosmetic, so I'm not sure things like ECB mode or absence of MAC really matter.

EDITED for clarity.

This seems like a choice made by some one who knows what they are doing. They are using AES-CBC for the actual conversation but for media they chose ECB. Of course having the media deleted on the client is their main selling point and they chose to use ECB there!

But ECB for media is particularly egregious, particularly since even the Wikipedia page on ECB shows how remarkably 'visible' large things encrypted with ECB are.

Hence the exclamation point in my response!

The key is a constant in the application, while ECB is embarrassing the mode can't help you here.

But if it makes you feel better you can upload arbitrary bytes to Snapchat, encrypt it with your own not-stupid method.