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I got it !

After some minutes searching in the av_shortcode portfolio.php i found the solution. For people who want the tips :

Line 419 in portfolio.php :

else
                {
                    $extraClass .= ' default_av_fullwidth ';

                    $output .= "<div data-ajax-id='{$the_id}' class=' rozepitem grid-entry flex_column isotope-item all_sort {$style_class} {$post_class} {$sort_class} {$grid} {$extraClass}'>";
	                $output .= !empty($excerpt) ? "<a href='{$title_link}' title='".esc_attr(strip_tags($title))."'><div class='rozeptxt grid-entry-excerpt entry-content' ".avia_markup_helper(array('context'=>'entry_content','echo'=>false)).">".$excerpt."</div></a>" : '';
                    $output .= "<article class='main_color inner-entry' ".avia_markup_helper(array('context' => 'entry','echo'=>false)).">";
					
                    $output .= apply_filters('avf_portfolio_extra', "", $entry);
                    $output .= "<".$link_markup[0]." data-rel='grid-".avia_post_grid::$grid."' class='grid-image avia-hover-fx'>".$custom_overlay.get_the_post_thumbnail( $the_id, $image_size )."</".$link_markup[1].">";
                    /*$output .= !empty($title) || !empty($excerpt) ? "<div style='z-index:100;box-shadow: 0px -5px 20px #888888;' class='grid-content'><div class='avia-arrow'></div>" : '';

                    if(!empty($title))
                    {
                        $markup = avia_markup_helper(array('context' => 'entry_title','echo'=>false));
                        $output .= '<header style="z-index:100;" class="entry-content-header">';
                        $output .= "<h3 class='grid-entry-title entry-title' $markup><a href='{$title_link}' title='".esc_attr(strip_tags($title))."'>".$title."</a></h3>";
                        $output .= '</header>';
                    }
                    $output .= !empty($title) || !empty($excerpt) ? "</div>" : '';*/
                    $output .= '<footer class="entry-footer"></footer>';
                    $output .= "</article>";
                    $output .= "</div>";
// Add a new div containing the new default image
	                $output .= "<div data-ajax-id='{$the_id}' class=' rozepitem grid-entry flex_column isotope-item all_sort {$style_class} {$post_class} {$sort_class} {$grid} {$extraClass}'>";
	                $output .= "<img src='http://www.dysign.fr/wp-content/uploads/2010/09/logo-dysign-2010-206x206.jpg' alt='pen'></img>";
	                $output .= "</div>";
// Close the div
                }

But now, how can i have to do to put this code in my child theme ?
Did i have to duplicate portfolio.php in my child-theme folder? or just duplicate the function called “function html” in my functions.php

Thanks for your answer