To add an image to your event page you have to use the attached images option on the add event page. Choose attached images at the top of the page, when you click on this a menu will drop down allowing you to browse for the image on your computer. After browsing to the image on your computer, put the race name in as the image title (this will make it easier to find if you want to reuse it for another page) then choose upload to upload your picture. When you save the page the picture will be on your event page.
This will make a preview size and a thumbnail size. The preview size will be used on your event page and the thumbnail size will used on the teaser page. You should start with a good image but if it is too big you may not be able to upload it. If it is too big to upload you can use windows paint too re-size your pictures.