Compiling on Epub, problems with OPF validator


I am compiling my book on ebook format for epub, with the cover image (an jpg) in the ebook file, but when I try to pass an OPF validator it gives me back this message:

Item ‘OPS/imágenes/portada.jpg’ exists in the EPUB, but is not declared in the OPF manifest.

I have checked everything when compiling, and it seems everything is ok, so I don’t know what else can I try.

Thanks for any help!