Hello Lee,
Try this snippet to display single/default image on detail page.
// Display single post image on detail page.
function _gd_snippet_detail_featured_image( $post ) {
	if ( ! empty( $post ) ) {
		$post_images = geodir_get_images( $post->ID, 'thumbnail', true, true, 1 );
		if ( ! empty( $post_images ) ) {
			$image = $post_images[0];
			?>
			<div class="geodir_flex-container" style="text-align:center">
				<img src="<?php echo $image->src; ?>" alt="<?php echo esc_attr( $image->title ); ?>" title="<?php echo esc_attr( $image->title ); ?>" style="max-height:400px;margin:0 auto;" />
			</div>
		<?php
		}
	}
}
function _gd_snippet_detail_single_image_slider() {
    remove_action( 'geodir_details_main_content', 'geodir_action_details_slider', 30 );
	add_action( 'geodir_details_main_content', '_gd_snippet_detail_featured_image', 30, 1 );
}
add_action( 'plugins_loaded', '_gd_snippet_detail_single_image_slider' );
Kiran