Best procedure for broken cached images?

[MarcC]

I changed the parameters for an image->size() operation on a site I'm running, and due to a loose pulley somewhere in the server, I now have a bunch of broken image icons in the website, where the thumbnail images should be. What's the best way to reset these--to start the resizing & caching operation again? I tried deleting one of the images from its folder in /site/assets/ but that didn't seem to do anything. I also tried resetting ProCache by saving a page. No change. I thought about changing the dimensions by one pixel but that seems kind of lame.

Thanks!