My understanding is that generating the Wang tile content is the most difficult part of using Wang tiles. All of the papers I've read use some manner of texture synthesis to build the tiles. It is possible to generate synthetic textures based on a source image using things like stochastic sampling, image quilting, & other techniques, but the quality of the output varies greatly depending on the combination of source image properties & the generation methodology. To the best of my knowledge, texture synthesis works best on abstract source images &/or highly repetitive source images. My academic experience with texture synthesis is that it can take a fair bit of tuning & tweaking to get desirable output.
@user1617735 mentioned the GPU Gems 2, Chapter 12, Tile-Based Texture Mapping, which in turn has this to say:
There are several methods to construct the Wang tile set. One possibility is to draw the tiles manually; all we need to do is to ensure that the drawings are continuous across tile boundaries while producing sufficiently interesting tiling results... This manual approach works only for line art.
For natural texture patterns such as those from photographs, there are algorithmic ways to build the tile set, as described in Cohen et al. 2003, Section 3.2. We have found that this approach works well for many natural texture patterns, and we provide the source code on the accompanying CD.