There seems to be a problem with memory usage during image processing in wp-photocontest/lib/thumbnail.class.php. I can get a fatal error after making one of the three thumbnails and the page returned is basically blank (without debug messages turned on in the admin panel for the plugin.) Consequently the entry doesn't get recorded in the DB tables.
Turns out it's running out of memory with the server memory-limit set at 30MB. The solution appears to be to find a hosting package with higher memory-limit - say 60MB. Or config your own dedicated hosting package. Trying to change the memory-limit doesn't work with init-set() and other options 'cos my provider won't allow it.
Trying a reload of the same image then fails because it already exists (in the full size version) on the server - a big user problem. Possible solution: document requirement that the server should have a memory-limit of at least, I don't know, say 48MB. I suspect most modern hosting packages may already have more memory available.
Other suggestions welcome, 'cos now I'm faced with having to migrate to a better hosting package.