Pictures of Lord Vishnu in various avatars (Dashavatar)
Vishnu took the form of a big fish. There is a story about this in Hindu scriptures. The Asuras or the demons snatched the Vedas, the holy books of the Hindus, and went deep inside the sea. At this point, Vishnu changed his form. He became a matsya, that is, a fish. He entered the sea and brought back the Vedas. There is another story of recreation of human kind after the pralaya, where the sage Manu put all the vedas and other important things in a boat and Vishnu took that to safety.
Here Vishnu took the form of a Turtle (kurma) and then goes the story of the samudra manthan or the churning of the ocean. He provided the base which helped in churning of the ocean. There is another story that when Demons got hold of Amrit, Vishnu helped the gods again in form of Mohini. One Demon had drank some Amrit, Whom Vishnu has cut into two. Thus Rahu and Ketu have been born.
Lord Vishnu takes the form of a Boar(varaha). Here the Lord kills the demon Hiranyaksha and saves earth.
Here the lord takes the form of half man and half beast. He assumes the form of lion headed man and proves how the little boy Prahlad was a true devotee.