These are two that epitomize "art on cardboard" to me, both commonly seen, but both worthy of mention.
The Mathewson's yellow and blue background shows the beauty of the lithography from the time period, while somehow capturing the grace and dignity of "the Christian Gentleman". (card shown is an E92, but the image exists in other E sets)
The Speaker is one of many pieces of art from the T3 Turkey Red set, showing Tris in mid-swing, most likely ready to unleash one of his 792 career doubles (yep, I looked it up!
). It's a still image, but you can imagine the fluidity and motion of the swing. The intensity in Speaker's face, the ripples in his uniform, and the red socks all come together to form a piece of art on cardboard.