Overlays can range from accent elements to word arrangements with a seasonal saying (as seen in the image), a border design of snowflakes to a bokeh light effect that can lightly cover an entire image.
Help your customer choose the overlay artwork based upon the image negative space, color, and look. Experiment with the placement of the artwork with the image to figure out the right place and size. Print off a color example or, better yet, sublimate an example on white so they can see the sublimated colors. Remember, what comes from your sublimation system will look different than your monitor or a color print-off.