Hi, only by modifying the implementation in /sections/portfolio_section_overlay.php or portfolio_section_slide.php where the portfolio pop-up items is constructed.

look for 

if( $post_layout == "image_popup" ){

currently the portfolio post title is displayed

thanks

Hello, the code you can find in /sections/portfolio_item_full_screen.php 

in get_featured_image_metadata function 

Use get_post_thumbnail_id($post->ID) to retrieve image id required as parameter.

thanks