Help to clear an error, please.

D2D epub validator is returning an error that doesn't make sense:

ERROR(RSC-001): .tmp.tmp3505da2u.epub(-1,-1): File 'OEBPS/image/letter_1.jpg' could not be found.

The file that could not be found doesn't exist, nor can I find any reference to it in any of my epub files. Can anyone suggest ways I might clear this error so my epub validates?

Thanks so much in advance.