Do you mean that your ex libris XHTML page has an <img> tag pointing to a JPEG image, which is shown in the Calibre viewer but NOT in Sony, iBooks, Bluefire, Stanza reader?
Is the image stored in the "ex libris"-ed eBook? (it should be in the exlibris/ dir).
Is the image really a JPEG image? (Maybe it is a PNG image, called image.jpg --- this situation might be problematic, since exlibris chooses the mimetype for the OPF file using the extension of the image file.)