However, I suspect it is easy to distinguish between the collection of least significant bits of a normal picture file and the collection of least significant bits of a picture file used to hold some encrypted data.
This may be. The connection is not obvious, but there may be correlations because of data conversions, mechanical scanner characteristics, etc. The first step in any such system which more closely hides data is to study carefully the statistics of base images. Until you understand them, any attempt at mimicking them is bound for failure. Seeking a good understanding of the statistical properties of messages of various sorts is generally missing in cypherpunks activity. The area gets quite technical, and we as a group need to develop some better understanding of it. Eric