A better solution to make image field into div background?
There is also the Background Images module suite:


  Background images is a collection of modules that allows users to add
  background images to html elements on pages of the site.


There is also a Field Group sub-module for background image support called Field Group Background Image:


  This module creates a field group display formatter that consists in a
   wrapper with a background.
  
  The background can be set from:
  
  
  An image field of the entity being displayed
  A color field (of Color Field or jQuery Colorpicker modules)
  
  
  This module can be used alongside
  Display Suite, and is particularly useful for creating custom
  Fieldable Panels Panes bundles.


You mention parallax scrolling ... I know what that is, I've never had to implement it -- you may want to try to use Parallex Background:


  This a simple module that allows to set a vertical Parallax effect on
  the background of any element on the DOM.


I kindly suggest you work more on your Google-fu skills.