I don't see any significant resemblance to Ryan. Anyway, the timing's a bit iffy. The book's publication date was August 1996, which means the cover art was probably done months earlier. And Dark Skies didn't premiere until September 1996. So would promotional photos from the show have been available early enough for the cover artist to see them? It's possible, but I'm not sure it's likely.
Plus Ryan wasn't part of the original Dark Skies cast (IIRC she first turns up around the middle of the season) so there'd definitely have been no photos of her and Walsh together (well, not from that show anyway) when this was done.