Find the Event page and select 'Edit', which will be shown at the top of the page. From here, navigate to the 'Extras' tab which appears in the sub navigation below the website header.
This will take you to the 'Images and extra information' part of the Event submission process, where you can add images and captions. Select the dashed square and locate the image files you wish to upload. Your image should load automatically.
Remember you must re-publish the event from the 'Preview' page in order for your changes to be saved.