It provides no pushback at all when a picture has been taken, jpeg encoded and then sliced into pieces and inserted onto the blockchain as extra data on a number of transactions, in a way easily recognised and easily re-extracts.

Sure, those numbers could have come about some other way, but they didn't.

Your hypothetical decoder is actually going to contain a very large amount of very specific data as well, so much so that the 'circle' part of it is more or less irrelevant. You've just defined a (very poor) data encoding scheme.