To protect in-flight data HTTPS encryption would be sufficient, since the attacker would have to MITM the connection and forge certificates instead of passively listening.

But once it reaches the end device it is impossible for snapshat to deliver on its promises of ephemeral messaging. To display the data the device needs to be able to decrypt it. If it can be decrypted it can be copied.

It's very much the same conceptual impossibility that DRM faces.

So in the end it doesn't really matter whether it's ECB or something better. It remains a DRM scheme.