[danbruc]

As already mentioned by others - hiding small amounts of data is easy, the challenge is to hide nontrivial amounts of data. There are algorithms that (try to) compensate statistical changes introduced by hiding data. One approach is to only use half of the available bit for hiding data and modifying the other half in a way to preserve a set of statistical properties.

It is probably not a good idea to hide data in images available on the internet because this enables direct comparison of the same image with and without hidden data.