I figured I was going to have to alter a SELECT; thanks for pointing me to the location.
Also, glad to hear you are going to add the "multiple deletes" as a feature request. In the meantime, I've looked at the table structure and see that in pg1_gallery_images there are columns for id, gallery_id and title that could be used to identify a specific image or even all of the images in a gallery. By examining the tables I get the impression that it should be OK to "manually" issue SQL DELETEs to directly remove image rows from this table. While I don't see any linkages to other tables (other than to pg1_gallery to id to which gallery an image belongs) I'd like to ask you to confirm that manually DELETing image rows from pg1_gallery_images would have no adverse effect on the system.
Also, you didn't address my question about having a large (say a hundred) number of Galleries and how the Gallery list might be grouped or filtered to make such a long list manageable. Am I correct in assuming that there are no facilities to help with this and my only option is to alter the code (probably in choosegallery.php) to add my own features to do this?