An artist can create a realistic figure using a model or a photograph (this is generally the case with paintings of nudes and commissioned portraits). However, it is also possible to draw, paint or sculpt a realistic figure without a model - purely from imagination, memory, and knowledge of human body proportions.
Two main things determine a figure: body proportions (and other anatomical features) and pose. The body proportions are determined by the person's age, gender, and individual physical appearance (e.g. tall/short, fat/slim...). The pose (including gesture) conveys what the person is doing or feeling. Facial expression adds to the emotional message of a figure.
Figures in spanking art
The characters found in spanking art are manifold. They include adults, children, fictitious cartoon characters, and humanoid animals. More or less any character is spankable if he/she/it has buttocks. However the majority of spanking art - maybe 98% - is either child spanking art or adult spanking art. The most common medium is the drawing and the most common styles are realism and cartoon.
Hobby or professional artists who want to draw spanking art will therefore need to acquire skills in figure drawing to begin with. A technique they may find helpful is described in the 3D Figure Drawing tutorial.
Alternatively, for the creation of rendered spanking art, an artist doesn't need skills in figure drawing, only a good sense for pose, 3D modelling, props, color, texture, lighting, and camera - many of these overlap with photographer's skills.