Welcome to mobileread ....
Have you looked in the epub forum (see main forum page)
epubcheck is not the be-all-and-end-all, its results are not always an issue, if a reader can read the files correctly.
looking at the error statement, and your comments that everything is there - are you correctly referencing the
paths to each item ?
[my suggestions do not imply that I know what I'm talking about
]