Display particular Image Style for image based on Role of User?
You can use a hook_preprocess_image_style to alter the style name to use:

/**
 * Implements hook_preprocess_HOOK for theme image_style.
 */
 function mymodule_preprocess_image_style(&amp;$variables) {
   $style = mymodule_determine_style_to_use($variables);
   $variables['style_name'] = $style;
 }


Take care that you do not alter all image styles, but only those you want/need to change, so mymodule_determine_style_to_use() should e.g have a check on $variables['style_name'] being S1 or S2.