If PNG is imported and over ca. 5000px of height it's not in highres anymore (in webview) but gets blurred by adobe XD: not showable, not usable and adobe seems to be not interested in that problem or would not fix it at all (perhaps even this is not a bug and just a way to keep other design-exports from sketch & co. out of XD?)

for now the only way to keep using the prototyping web-view is to slice our pngs in smaller peives and play puzzle in xd... WOOOW!