The name Isis means "(She of the) Throne". Her headdress is a throne. As the personification of the throne, she was an important representation of the pharaoh's power. The pharaoh was depicted as her child, who sat on the throne she provided.
In art, originally Isis was pictured as a woman wearing a long sheath dress and crowned with the hieroglyphic sign for a throne. Sometimes she was depicted as holding a lotus, or, as a Sycamore tree.
After she assimilated many of the roles of Hathor, Isis's headdress is replaced with that of Hathor: the horns of a cow on her head, with the solar disk between them. Sometimes she also was represented as a cow, or a cow's head.
In The Book of Coming Forth By Day (ancient Egyptian funerary text) Isis is depicted standing on the prow of the Solar Bark with her arms outstretched. Hence the above depiction of her. :)
Registration for my Cotton Candy Girls On-line Workshop is now open. Click on the banner to take you there. :) Check out my GIVEAWAY to win a free spot on my e-course.