I have two designs in mind but I will give you three options:
Option 1: The cover should show the robot and the girl in the center of the cover walking towards a ruined, post apocalyptic suburban neighborhood during a bright and starry night. They're holding hands, with the little girl looking up at him, and the robot looking down at her. The little girl is carrying a dirty stuffed bear in her other arm. The girl is 8-10 years old, and has been wearing the same dress for a couple weeks at this point. She has brown hair. For the robot, think along the lines of the movie I, Robot, except instead of white and blue, he's blackish/greyish and blue. He's an elegant humanoid robot meant to be lightweight. And he should have signs of age and battle wear: things like chipped paint or faded areas, battle marks and such, let your creativity go with this.
Option 2: This option for the cover will strictly focus on the little girl's hand holding the robot's hand. The girl's hand should be significantly smaller than the robot's, and the robot's hand should be metallic but human like (meaning five fingers and a palm). You can choose to show a little bit of the girl's arm and shoulder area with a little bit of her hair, and you can choose to show a little bit of the robot, around the waist/upper leg area. But only if you think this adds to the overall intimacy of the hands holding.
Option 3: Create something entirely of your own creativity and originality. Keep in mind that the first two options are what I have in mind and what I'll probably be biased towards, but if you think you have an idea that is more appropriate and will blow me away, go for it!
For the font: I'm thinking big white letters at the top that read "The Girl And The Robot" with "The Girl" having an elegant font, something like italics or cursive, and "The Robot" having something more militarized or robotic. 60 pt font or higher. And author name "Ernie Luis" at the bottom.
Image should be a JPEG with a height/width ratio of 1.6. A minimum of 625 pixels on the shortest side and 1000 pixels on the longest side. For best quality, your image would be 2820 pixels on the shortest side and 4500 pixels on the longest side.