Even a 15mb TIFF gets downgraded a bit on Scrivener and through another forum about images where people were talking about cover images for ebooks, they had visual comparisons of PDF images that do not get downsampled by the importing program.
I have personal experience from printing 4ftx8ft maps from GIS wherein exporting to PDF is indeed the best way to preserve image quality as well,
So I have imported a PDF image of my cover, when I click on it in the image folder, I can see it just like any other image, it is the correct size and I don’t see any quality loss.
However when I am at the compile menu, it does not show up as an option to be selected to use as my cover.
Is there a way around this?