1. Backup the original files before making any changes.
2. Open the file wp-content/themes/event-manager/single-sc-speakers.php
3. Find (at line 23)
echo ‘<p>’ . get_the_post_thumbnail( $post->ID, ‘sc_thumbnail’ ) . ‘</p>’;
4. Replace with:
echo ‘<p>’ . get_the_post_thumbnail( $post->ID, ‘full’ ) . ‘</p>’;
6. Update the file.