Thumbnail in Mashup Marker

Hello,

how can I put my Thumbnail into the mashup map?
I have this image code.

the_post_thumbnail('venue-thumbnail')

When I use this filter

function prefix_pgmm_item($itemContent) {
	$itemContent = '';
	$itemContent .= '<a href="'. get_permalink() .'">';
	$itemContent .= 	get_the_title();
	$itemContent .= '</a>';
	$itemContent .= '<br />';
	$itemContent .= the_post_thumbnail('venue-thumbnail');

	return $itemContent;
}

add_filter('pronamic_google_maps_mashup_item', 'prefix_pgmm_item');

the thumbnails show up double above the map 🙁

How can I fix that?

Cheers,
Denis

http://wordpress.org/plugins/pronamic-google-maps/