link to post, and noimage instead of default

Hi

I use the_thumb() along with the_excerpt in archive view. I would like thumbnails to link to their respective posts in stead of linking through to the image source – as the default thumbnail does when there aren’t ay images in the post –

And when there are no images in the post I’d rather show nothing than a default image

Using it at e.g. http://www.davidkrutpublishing.com/dkp/performance-media/