I guess fan artists are well-versed in Western art history. And also need to be creative about their photo references, given the dearth of nude pictures of William Shatner floating around.
Something interesting going on here gender-wise, putting Kirk in the pose of a girl gazing up at the stars. Kirk has already been there! In canon he is the manly space hero, but in this piece he somehow looks more vulnerable and passive than the girl who just happens to be hanging out on a rock on the ocean, naked, as you do. She’s probably making plans to hop on a spaceship as soon as she finds some clothes. Meanwhile, Kirk seems to be agonizing over something (Spock?), and he doesn’t look too optimistic about his prospects.