I'm not attaching files -- at least intentionally, 'cause the new forum software seems like it's uploading files automatically -- but rather trying to attach via URL link only. ISTR Ryan mentioning something about the forum archiving all images now so they'll be preserved for use here.
Linking smaller display sizes hasn't helped. But that doesn't change my original SmugMug file size, of course, just the size displayed by a particular URL.
Dunno... It was still happening yesterday, and using the workaround of adding the photos through successive edits to the original post seems to work.