Go to the Event.Gives Manager: manager.event.gives. Login using your cell phone number and password. Once you are logged in click "campaigns" and then select the campaign you'd like to edit.
After logging in, you can now edit anything on your page. This includes the title, description, and the image you used.
Images are best if they are right centered (because your campaign details cover part of the page).
The image you use for your campaign needs to be 1920 x 1080. It's more compelling to see an actual photo than a logo.