These are hypothetical covers for imaginary movie novelizations. They're classier than actual covers, if you ask me. Compare this Back to the Future cover...
...with this actual cover for the novelization of Back to the Future III, which I just happen to have:
The hypothetical one has a distinct lack of Christopher Lloyd and Michael J. Fox being plastered across the cover.
Also, now I'm wondering if there was a novelization of the original Highlander movie. It seems like it came out at just the right time for one, but it'll be hard to search for with all the Highlander TV Show tie-in novels that have come out.
(UPDATE: It turns out there is! Oh boy!)