image field point to another image field
What you need is a change in the way the content is displayed, so it is much better to do it in theme layer rather than content level.

I would recommend using template preprocessing for node.

Add this function to the template.php file in your theme folder:

&lt;?php

/**
 * Implements hook_preprocess_node().
 * 
 * Replaces the node's image field with anbother node's image field
 */
function YOURTHEME_preprocess_node(&amp;$variables) {
    $node = $variables;
    // Make sure we only alter nodes that we intend to!
    if ($node-&gt;type == 'article') { // assiuming your article content-type is 'article'
        // Get the view mode for this node, e.g. full or teaser
        $view_mode = $variables['view_mode'];
        // Get the group node
        $another_node = get_another_node($node); // Somehow!
        // Retrieve the image field of that node using the same view mode of the current node
        $another_image_field = field_view_field('node', $another_node, 'field_image', $view_mode); // assiuming it is called 'field_image'
        // Replacing the other field with the current node's field
        $variables['content']['field_image'] = $another_image_field; // assuming it is called 'field_image'
    }
}


You will need to update the code to retrieve the node object for the group and also make sure that the field names are correct for your case.