I’m compiling to LibreOffice directly from Scrivener and while jpeg images show up in the LibreOffice document they do not convert to PDF or even under Page Preview. When I compile directly to PDF the images DO show up.
The same is true when I compile to MMD and open in MMD.
If the images are present in the LibreOffice document, and in a PDF compiled directly from Scrivener, it seems to me that Scrivener is doing its job. What LibreOffice does with the images after that would be a question for them to answer.
Another test would be to compile to RTF from Scrivener, open the resulting file in TextEdit, and see what happens with TextEdit’s Print to PDF command.
One slight correction: TextEdit cannot actually load RTF graphics, so that test will always fail. It uses Apple’s own proprietary RTFD format for images, which we do support, but that probably wouldn’t be a good test for RTF image embedding since the technology is completely different.
It could be something with the JPEG files themselves. What happens if you import one of the files into a test LibreOffice document and try to create a PDF with it? I just ran a quick test with a PNG and JPEG file, compiled from Scrivener to RTF, opened in LibreOffice and exported a PDF with no problems.