return the title of a thumbnail with get_post_thumbnail_id

Does anyone know how I can return the name (i.e. Title) of an image being used as a thumbnail with the new Page Image feature?

I can return the ID using:

get_post_thumbnail_id($post_id);

but I want to return the name of the image so I can replace a part of it and then use that to call a different, yet similar, image.

Could be I’m using a clumsy-thumbed method to achieve what I want but *blush* it’s the only way I know how.