I haven't had consistent results using this plugin to insert or replace covers recently. Sometimes it works, sometimes not. This is the log from the latest try:
Spoiler:
calibre, version 0.8.56
ERROR: Modify ePub failed: No ePub files were updated
Modify ePubs
Logfile for book ID 7934 (Rachel's Totem / Harte, Marie)
7934
Modifying: C:\Users\Rebekah\AppData\Local\Temp\calibre_0.8.56 _tmp_wufedv\paleuz_modify_epub\7934.epub
Adding or updating jacket
Manifest item added: jacket.xhtml (calibre_jacket)
Spine item inserted: calibre_jacket at pos: 0
Insert or replace cover
...Looking for guide cover reference
Found guide cover reference to: cover.html
Related manifest item id: cover
...Looking for meta cover
Found meta tag with cover id: cover-image
Meta item removed: cover
...Looking for other.ms-* guide references to clean out
...Working with possible existing cover: cover.html
Guide reference removed: cover.html
...Looking for TOC navpoint for this cover
...Inspecting old cover for removal
Cover page contains only this image so can be deleted
Manifest item removed: cover.html (cover)
Spine itemref removed: cover
Checking safe to remove cover image: OEBPS/images/cover.png
Rachel's Totem - ERROR: Traceback (most recent call last):
File "calibre_plugins.modify_epub.modify", line 62, in process_book
File "calibre_plugins.modify_epub.modify", line 175, in _process_book
File "calibre_plugins.modify_epub.modify", line 703, in _insert_replace_cover
File "calibre_plugins.modify_epub.covers", line 63, in insert_or_replace_cover
File "calibre_plugins.modify_epub.covers", line 296, in _remove_old_cover_if_safe
File "calibre_plugins.modify_epub.covers", line 253, in _find_item_for_cover_image
File "calibre_plugins.modify_epub.container", line 751, in get_page_image_names
File "calibre_plugins.modify_epub.container", line 209, in get_parsed_etree
File "calibre_plugins.modify_epub.container", line 197, in get_raw
KeyError: u'OEBPS/jacket.xhtml'
ePub not changed after 0.37 seconds
Can someone tell me why this didn't work?