[Plugin: RSS Custom Fields] Additional custom field

Hi,

thanks for the plugin!

How can I add an other custom field named “Images” showing the image thumb to the code?

Without success I tried

add_post_meta($post_id, ‘Image’, ‘[Image path]’, false);

Is there a special place in the code where I have to add it?

Thanks!
Boris

http://wordpress.org/extend/plugins/rss-custom-fields/