You can upload a .jpg image to use as the splash image for your player by visiting Player Options -> Go Player -> Advanced -> Splash Image in the Sunday Streams Manager. From there, you can remove your current splash image, or upload a new image. It is recommended to upload an image that is in the 16:9 ration (e.g. 1920x1080, 1280x720, etc.).
NOTE: You can also upload a logo image and a background image for your Go Player by accessing the "Logo Image" and "Background Image" sections as well.