What is a thangka?
A detailed painting of a Buddha or Bodhisattva that they look at before they meditate.
What is the medicine buddha?
They may visualise him to help heal them and reduce their suffering
Who is avalokiteshvara?
a Buddha who is related to compassion, visualising him will help them develop their own sense of compassion.
Why do Buddhists visualise amitabha buddha?
To hope he will help them be reborn into the Pure Land and achieve enlightenment quicker.