What you want is called a masking/stencil film. You can spray the area of the psp that you want the image with the color you want for your image, cut your image out of the masking film, apply it over the paint (after it has cured), then paint the rest of the psp whatever color you want for the 'background', then peel the masking film off the psp, when ready spray a clear coat over the whole psp.