Lord Vishnu takes various avatars (Dashavatar or Dashavatharam) whenever the world is threatened with the evil forces. The most prominent of his incarnations (i.e., avatars) are Lord Krishna in the epic Mahabharatha and Lord Rama in the epic Ramayana.
The other common names for Lord Vishnu are Narayana, Jagannath, Vasudeva, Vithoba, and Hari.
In Hindu iconography, Vishnu has always been depicted as having a dark/pale blue complexion body with four arms. He holds a lotus flower (Padma) in his lower left hand, Kaumodaki gada (mace) in his lower right hand, Panchajanya shankha (conch) in his upper left hand and the Sudarshana Chakra (discus) in his upper right hand.
A traditional depiction is Lord Vishnu reclining on the coils of Ananta, accompanied by his consort Goddess Lakshmi.
Here is a huge collection of Lord Vishnu Images including HD wallpapers:
Lord Vishnu Images
Lord Vishnu and Lakshmi Images
Lord Vishnu HD Wallpapers
God Vishnu In Other Forms Gallery
Lord Vishnu Rescuing Elephant From Crocodile Images
-  – “An Introductory Dictionary of Theology and Religious Studies.” Google Books. Accessed December 31, 2016. Link.
-  – “An Introduction to Hinduism.” Google Books. Accessed December 31, 2016. Link.
-  – “Gardner’s Art through the Ages: Non-Western Perspectives.” Google Books. Accessed December 31, 2016. Link.