Essays / the Arthurian legend - critical analysis [2]

Hey im in yr 11 and doing extension 1 English in Australia. We are studying the Arthurian legend and looking at it from different perspectives and have been ased to write a critical anylisis on three pictorial representations of the legende:one medieval, one victorian and one from the 20th centuary.In the anyllisi we are to show how the meaning of the legned chances with context and anylisis color, composition etc... Firstly what is a good introduction. Can you give me an example of how to start a critical analysis and the what is the structure of this analysis and some tips.

please help.. im really stuck.