Hey Michael,
Would this be for a specific list on a page with multiple lists?
If so, you could try:
1. Changing your shortcode to include a custom location:
[wp_show_posts id="123" settings="image_location=below-content"]
Then add this function:
add_action( 'wpsp_after_content', function( $settings ) {
if ( 'below-content' === $settings['image_location'] ) {
wpsp_post_image( $settings );
}
} );