Hesperus, the Evening Star, Sacred to Lovers, Joseph Noel Paton; oil on millboard, roughly 36 x 27 inches (91 x 69 cm). Link it to zoomable image on Art Renewal. There is a larger downloadable file on Arthive.

Though not actually a member of the group, Scottish painter Joseph Noel Paton was loosely associated with the Pre-Raphaelites, and was a friend of Millais.

Hesperus, in Greek mythology, is the personification of Venus as the Evening Star, which we see through the tree branches above the lovers.

My experience with Pre-Raphaelite and other Victorian paintings makes me believe there is symbolic significance to the particuar flowers and fauna around the couple, but if so, I don’t know what the reference is in this case.