How can I write a caption on the back of the attached book cover that I designed for the short story "The Ship Who Sang" by Anne McCaffrey?

To provide a caption for this picture, you should credit its source. You can also provide a short description of what is going on in the picture. This is up to you, but it can be something along the lines of, "Helva, a brainship, has the energy and brains to travel through space." 
The back cover also usually provides a short summary of the novel or story. You may want to include something along the lines of the following: "Helva, a cyborg, becomes a 'shell person,' living her life beneath titanium as her superior brain pilots her spaceship. She is not an ordinary person, but she still struggles with human emotions. Though she is by nature not a defeatist, she goes on her journeys with a sense of loss after her lover, Jennan, is killed, and with worries about whether she'll ever be able to pay off the considerable debts that she owes for her training. Will she ever find the right brawn to partner with her and to provide her with the companionship she has long wanted?" (The short story stops a bit short of including her continued search for a new brawn, so you can cut that part if you are writing only about the short story and not the novel.)