Black and White Thumbnail in WordPress Gallery

I think the best way to do this would be to add a filter to when the content is pulled into the post/page:

<?php add_filter( 'the_content', 'add_bw_thumbnails' ) ?>

Or to when an image is inserted into the editor:

add_filter( 'post_thumbnail_html', 'add_bw_thumbnails', 10 );
add_filter( 'image_send_to_editor', 'add_bw_thumbnails', 10 );

You then just have to write the function to do the replacement. Do you want to do this for all pages, or only those in a gallery, and are you sure 150×150 is the right thumbnail size?