[scribus] pxd/x3 export : apparently contains RGB images

[eric hanuise]

Hi all.

I've exported a PDF as pdf/x3 using isocoated v2 ICC profile.
It contains several png images, which are rgb.

As I understand it, the pdf exported by scribus should be cmyk only, and 
the images converted to cmyk during pdf generation.
however, the printer came back to me and told me the pdf contained RGB 
images.

Have I misunderstood the ability of scribus to convert everything to 
cmyk on pdf export, or is there some setting I missed ?

Thanks in advance for your help.