To do this, go into the Promotion -> Social Sharing page on your race dashboard. You can upload a large image (it needs to be 1200 X 630 pixels to conform to Facebook’s requirements) and/or a small image (382 X 200) for the image to appear on the side. We recommend the larger image as the default to help get your brand widely seen.
Once these images are in place, when participants share the fact they registered for your race, it will show on their timelines with a great looking image for your race.