I disabled static html caching before posting this...

The album shows no thumb for this image (See the "Javera Ebonydream" link at the end):
http://davenporthorses.org/photos/Modern%20Horses/page/3/

The image page shows a link rather than the full photo:
http://davenporthorses.org/photos/Modern%20Horses/ebony10.jpg.php

Clicking through to the photograph shows the full photo, so it's not a permissions issue:
http://davenporthorses.org/photos/Modern%20Horses/ebony10.jpg.php?z&p=full-image

I tried deleting the photo via the admin interface and copying it in at the shell level rather than uploading. Still an issue. Reloading the album page doesn't help. Neither does pre-caching the images in that album, or using the "crop thumbnail" link.

So it is probably a processing error, then. Not sure what it would be, but you would need to review the CGI logs to see what error you are getting.