Use the announcement image as the icon - Props Pete Nelson

gistfile1.txt

/*
* Timeline Express Custom Icon Filter Test
* Note: Requires Timeline Express v1.2 or later.
*	The following code should be placed in the bottom of your active themes functions.php file. 
*/

add_filter( 'timeline-express-custom-icon-html', 'pn_timeline_express_custom_icon_html_test', 10, 3 );
function pn_timeline_express_custom_icon_html_test( $html, $post_id, $timeline_express_options ) {

	$custom_png_icon = get_post_meta( $post_id, '_custom_png_icon', true );

	if ( ! $custom_png_icon ) {
		return $html;
	} else {
		$image_html = '<img class="custom-image" src="' .$custom_png_icon . '" />';
	}

	if ( empty( $image_html) ) {
		return $html;
	}

	// capture custom image HTML for the icon
	ob_start();

	if ( $timeline_express_options['read-more-visibility'] != 0 ) { ?>
		<a class="cd-timeline-icon-link" href="<?php echo get_the_permalink( $post_id ); ?>">
			<div class="cd-timeline-img cd-picture cd-timeline-png">
				<?php echo $image_html; ?>
			</div> <!-- cd-timeline-img -->
		</a>
	<?php } else { ?>
		<div class="cd-timeline-img cd-picture cd-timeline-png">
			<?php echo $image_html; ?>
		</div> <!-- cd-timeline-img -->
	<?php }

	$html = ob_get_contents();
	ob_end_clean();

	return $html;
}

add_filter( 'timeline_express_custom_fields' , 'add_custom_timeline_express_field' );
function add_custom_timeline_express_field( $custom_fields ) {
	$custom_fields = array(
		array(
			'name' => __( 'Example Text Field', 'timeline-express' ),
			'desc' => __( 'this is an example user defined text field.', 'timeline-express' ),
			'id'   => '_custom_png_icon',
			'type' => 'file',
		),
	);
	return $custom_fields;
}

add_action( 'wp_footer', 'pn_timeline_express_custom_icon_html_footer_css' );
function pn_timeline_express_custom_icon_html_footer_css() {
	?>
	<style>
		#primary .cd-timeline-img {
			border-radius: 0;
			box-shadow: none;
		}
		#primary .cd-timeline-img.cd-picture {
			background: none;
			border: none;
		}
		#primary .cd-timeline-img img {
			width: 60px;
			height: 60px;
			margin-left: -30px;
			margin-top: -30px;
			height: auto;
		}
	</style>
	<?php
}