The agent card/ agent unit is located in templates/agent_unit.php

In there we read the agent details and we display the information. Agent details like phone or mobile are saved as post meta and can be loaded with get_post_meta

$agent_mobile       = esc_html( get_post_meta($post->ID, 'agent_mobile', true) );

Agent image is saved as post attachment. It  is loaded at line 23 via

$thumb_prop    = get_the_post_thumbnail($post->ID, 'property_listings',$extra);

If no image is found we load a default image

if($thumb_prop==''){
$thumb_prop = '<img src="'.get_template_directory_uri().'/img/default_user.png" alt="agent-images">';
}