Bob03 on "Most images display; some don't"

I've installed Zenphoto (v 1.4.12) from scratch on a new host and I'm uploading my images into sub-albums via FTP. I have a problem in that the Album page displays all the thumbnails correctly, but while most of the images show correctly on the Image pages, there are several that don't. These show just the image title as a link. Clicking this link opens the full-sized image. It's the same whether I click on the thumb-nail from the album page or whether I page through the images. No amount of refreshing has any effect. Deleting and re-uploading the images has no effect. All the images are shown correctly on the Admin > Albums > Images tab. I'm using a customised version of the Basic theme but the problem is still there if I revert back to the un-modified theme.
Any suggestions what could be wrong?