Why does wordpress template seem to negate html code?

     

Ive been creating websites in html/css and by help of wordpress sometimes. Talking of wordpress templates there are some concepts which i do not grasp.  Wp templates are made of php files and have their own coding like instead of


  

i have to write following to include an image:


  

why doesnt simple html code not work and why for example does following tweak of the wp template code not work for me(to center image) ?


  

Obviously im not that familiar with wp templates codes as i would like to be is there a comprehensive literature to gain this knowledge (the wordpress codex page is not comprehensive enough imo and at my level

  
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answer1
  
  

Provided your containing element is wide enough and has the correct display attribute