You will need the Full Write Events permission or be an organizer on the event to perform this action.

Navigate to the event you want to upload/change the image for. A little lower on the right hand side, there is a box containing the current event image. Where indicated, you can drop a new image file or click on the "Browse..." button to search for the image on your hard drive.

Once uploaded, the image will appear under the "Info" tab. The recommended image size is 1600x500. However, if you upload an image smaller or larger, the system will resize as needed while maintaining the original aspect ratio.