Digital imaging devices have interesting pixel patterns. A common pattern found in digital cameras is four sensors in a square grid, red/green/blue/green. Since most image formats, like PNG, assume only three channels centered on one another, the four raw pixels must be interpolated to create one "cooked" pixel.

Due to reasons relating to signal processing which I'm not qualified to discuss (e.g. Nyquist limit and aliasing), a naïve sampling of these pixels can result in image artifacts such as Moiré patterns. My guess is that the artifacts I'm seeing in these images are of this type, but I could be wrong.