[scribus] Question about "Opacity"

[Frank Pieczynski]

Hello,
I have a question about usage of "Opacity" in 'Properties'->'Colors' of a image:
My goal was a light big background picture on a flyer. So I opened a
image frame, put it to background and set the 'Opacity' to 70%. On top
of that I placed the text frames and other images.

BUT during PDF export scribus gives me an red error for the background
frame! I decided to ignore the error and the result was as expected
and looks beautiful. What's wrong or what is the right way to do this?
I tried with 'Image effects', but was not able to get the same
results.

Thanks,
Frank