Compare original vs BPG in the soccer picture, then look at the green strip on the shirt of the red team player and slide over it. It's magical how perfectly the compresion make the red dots disappear.
Another great one is the Irish Mannequinn. I thought BPG blows up the edges, because every other format smoothed them out. But no, when you compare to the original, all the interesting edges (hair, hanging fabric, top of the lips) should be very visible.
That looks like an aggressive chroma subsampling/quantization.
Image encoders must allocate the bitrate among the channels, luma (Y), and chroma (Cb, Cr). Most of the time it is better to allocate most of the bitrate to the luma, because that's what the eye is most sensitive to. However it can create artifacts like this.
