Short answers, pulled from the story.
The Tjängvide image stone is a Viking Age limestone slab carved between approximately 700 and 900 CE, discovered in 1844 on the farm of Tjängvide on the Swedish island of Gotland. It is now held at the Swedish Museum of National Antiquities in Stockholm.
The rider is most commonly identified as Odin on his eight-legged horse Sleipnir, arriving at Valhalla and welcomed by a valkyrie with a drinking horn. An alternative interpretation, based on the Volsunga saga, identifies the rider as Sigurd on Grani, welcomed by Brynhild or Grimhild at the court of the Gjukungs.
The surviving inscription, catalogued as Gotland Runic Inscription 110 in the Rundata project, reads that someone raised the stone in memory of Hjorulfr (or Jorulfr), described as his brother. The name Hiorulf translates as sword wolf. Many runes on both sides of the stone are lost.
The lower field of the Tjängvide stone is almost completely filled with the image of a longship with tall aft and stern, its sail nearly as wide as the ship is long. The stone's lower field is separated from the upper scene by a braided pattern resembling valknuts, figures associated in Norse imagery with Odin and fallen warriors.
The Tjängvide image stone is considered pagan in origin. No trace of any Christian elements has been found in its inscription or imagery, and its iconography connects to Norse mythological and heroic traditions.
The Swedish Museum of National Antiquities in Stockholm, where the stone is held, uses the image of the rider on the eight-legged horse from the Tjängvide stone as its institutional logo.