I would say there is no way you can prevent users from copying the data and sending it on unauthorized. Making sure the data is only available as images on the client is one solution, but they could still take a screenshot and distribute that. The approach from the article seems overkill in most cases (they indicate themselves that it is slow - I guess plain SSL is a lot faster). In any case, I see no possibility to automatically enforce a limited use of data, contracts only. In that sense watermarking can help trace who distributed the file if you include a different watermark for each user.