title_body: Are image subfolders allowed in Sigil?
upvoted_answer: After doing more research, I've learned that Calibre allows subfolders in the Images folder but Sigil doesn't. There are some suggestions that you can tweak Sigil to allow subfolders, but I haven't verified that it can be done.
  
  https://www.mobileread.com/forums/showthread.php?t=283132