This may clear up after the site moves, however:
Image attachments are always full-size, even if "Thumbnail" is selected when attaching them. I tried to modify the BB code to force a size, but that did not work, resulting in the code being explicitly displayed instead of the image. From poking around on Stackoverflow, it seems this is an issue that has been reported by other BB code users on various forums, but no solutions were offered (from what I could find in a brief-but-not-too-brief search.)