As per summary. I guess the problem is that raster images in tiff are actually encapsulated JPEGs. Probably worth adding REQUIRED_USE.
jpeg is just one option for the internal tiff storage: http://en.wikipedia.org/wiki/TIFF the ebuild was just missing a knob or two: http://sources.gentoo.org/media-libs/netpbm/netpbm-10.56.00.ebuild?r1=1.1&r2=1.2