samps, just for a bit of extra info I tried your workflow and got the following results:
On OS X 10.9, Scribus 1.4.3:
Adobe Reader 9.5.5 - PDF looks as expected, rectangle and image (OKAY).
Preview - OKAY.
Skim 1.4.2 - OKAY.
Firefox 25.0.1 - OKAY.
Safari 7.0 - OKAY.
Chrome 31.0 - OKAY.
Chrome Canary 33.0 - OKAY.
On Ubuntu 13.10, Scribus 1.4.2:
Evince - Image is missing.
ImageMagick (display) - OKAY.
GNOME Document Previewer - Image is missing.
MuPDF 1.2 - OKAY.
Firefox 25.0.1 - OKAY.
Chrome 31.0 - OKAY.
It looks like there are some differences between viewers but most show the document just fine when generated from stable Scribus versions.
If you can find a reader that displays the document without issue then I’d say it might not be a bug with Scribus but, in this case, there is a chance that Scribus isn’t quite creating the PDF correctly, which would be a bug, if it wasn’t properly following the rules.
So, I’d say that utnik’s suggestions are both perfectly valid. Try using a stable version first - if at all possible - then, if you still get the problem, file a report - if you can - and let the developers see if it’s a bug or not (they are best placed to decide).