Naor and Shamir [NS94] developed what they called visual secret sharing schemes, which are an interesting visual variant of the ordinary secret sharing schemes.
Roughly speaking, the problem can be formulated as follows:
There is a secret picture to be shared among n participants. The
picture is divided into n transparencies (shares) such that if any
m transparencies are placed together, the picture becomes visible,
but if fewer than m transparencies are placed together, nothing
can be seen. Such a scheme is constructed by viewing the secret picture
as a set of black and white pixels and handling each pixel separately.
See [NS94] for more details. The schemes
are perfectly secure and easily implemented without any cryptographic computation.
A further improvement allows each transparency (share) to be an innocent
picture (e.g. a picture of a landscape or a picture of a building), thus
concealing the fact that secret sharing is taking place.