I've either used the administration tools to delete the image, or I simply move it out of the catalog folder and then move it back using a FTP tool. If I use the latter technique I then reload the main catalog in a separate Firefox tab. Using either method, I then restore the old comments by hand, using cut and paste from a separate text file, and then re-tag the image. Either deletion method seems to give the same effect.
As the number of images in the db are small, if necessary I can simply regenerate all the images and text files, and hope that whatever I did before to cause this problem doesn't happen again. I don't want this to turn into a time sink for anyone, as I may have cause this weird problem merely but uploading the same images more than once or some such error, which I'm unlikely to repeat again now that I've set up a better way to edit my php files offline.