We've created this Forum Thread with some workarounds and advice to assist you in publishing you ebook.
Cannot upload cover image
Tried everything for 6X9 hardcover casewrap. Tried a fraction under as well as over 6.875x10.75 total measurement. Confirmed 300 ppi. No success with jpg, tried pdf and tif. One screen indicated my file size (6.7-6.9 Mb) was OK. I even tried a different pc. All attempts were rejected without a word of explanation except "file too large" (tif) or (file corrupt (pdf). I never had a problem in this step with 2 previous paperbacks. No reponse yet from lulu support and no phone number to call. Thanks for any help.