To add images and/or a YouTube video on your fundraiser project, login to your fundraiser account and view your project page as your donors would see it (My Projects > click on the name of the project).
To add a cover image at the top of the page, click on "Edit cover" above the project title. Browse 1 image and when done, click on "Save changes".
To add gallery images and/or a YouTube video below the Story section, click on "Edit gallery" below Gallery. Browse images and/or add a YouTube URL and when done, click on "Save changes".