Mark,
I’m pretty sure we’re using NextGEN because the images are inserted with the shortcode (i.e. [gallery ids="2559,2560,2564"]). It looks like its adding each image as a separate gallery rather than just a single one. The problem also seems to be related to the fact that I can’t add a new gallery in the nextGEN menu on this particular blog. Other blogs in the network work fine.
MK