Both PNG and TIFF can be 16 bit, jpg can only be 8 bit. Have you checked if perhaps your png and tif file use 16 bit? Perhaps it's that what Poser can't handle.
Edit: And yes, what English Bob said. Don't know what color space you're using in PS. If it is some special setting, that might be embedded in png and tif but jpg doesn't support that, so that might be another reason for a difference.
Edit: And yes, what English Bob said. Don't know what color space you're using in PS. If it is some special setting, that might be embedded in png and tif but jpg doesn't support that, so that might be another reason for a difference.