So, a thumbnail-size insert on the book's back cover informs us that the stunning front cover of Murder of a Lady is taken from an old advertisement poster for Tarbet Hotel on Loch Lomond. (This isn't the only book where the British Library does this sort of mini "front cover source reveal" on the back, and I love them for that alone!)
That being the case, I figured I'd add some (more or less) present day visual context from a visit to the area some years ago, since Mr. Wynne informs us in the book's very first line that we are, indeed, in Argyll, so the selection of the British Library cover is not a random pretty picture from the Scottish Highlands but one striving for a certain level of period authenticity.
Tarbet Hotel aerial views (on YouTube)
(Note: When in BL Dashboard view, click on the link -- the video insert only shows in blog view.)
Though we didn't visit Tarbet during our trip, we did see several similarly fine old buildings on the shores of Loch Lomond; typically private properties whose access is barred to the general public, but which you can at least view from afar during a boat trip on the loch (or when climbing one of the nearby hillsides).
... and here's some more of the Loch Lomond scenery (with Ben Lomond [= Mt. Lomond] figuring prominently in the last two pictures):