Short answers, pulled from the story.
A picture stone is an ornate slab of limestone from ancient Northern Europe, primarily located on the island of Gotland. More than four hundred such stones exist today scattered across Germanic Iron Age and Viking Age Scandinia.
The practice emerged between 400 AD and 1100 AD marking a distinct cultural moment in Northern Europe. The first group appeared between 400 and 600 AD while later examples developed between 700 and 1100 AD.
Unlike runestones which rely on written text these monuments convey their message through carved images alone. The lack of runic text means the images had to carry the full weight of the message for public communication.
Archaeologists have found them in grave fields though rarely placed directly upon a burial site itself. Some stones stood at bridges or along roadsides so that many people could see them during daily travel.
One famous example from Tjängvide image stone shows Odin entering Valhalla riding on Sleipnir. Other carvings depict sacrifices battles taking place within the stone's surface fields and figures holding snakes known as the Snake-witch stone.