[Scribus] Images in resulting pdf

[KS]

Hello,

I have created the pdf of the brochure I'm working on. The are several
images in the brochure and they look very nice when viewed as images.
However, after the conversion to pdf (JPEG compression of Max quality at
original dpi of imgages) there is a lot of black level in the images. It
is as if the image was scanned by a bad scanner unable to distinguish
slight differences in the levels.

Is there anything I can change to get a better quality output?
Thanks,
/kds

PS: The original images were png graphics.