It's likely that the Pressbooks ad you're seeing is actually a filler ebook cover supplied by Pressbooks until you've uploaded your own.
Most ebookstores require that your cover image is included in your EPUB. You can insert your own through the Book Info page in Pressbooks.
For more information on how to insert your ebook cover and get rid of the Pressbooks default cover image, check out https://guide.pressbooks.com/chapter/how-to-design-your-book-cover/. Scroll down to the "Uploading Your Ebook Cover in Pressbooks" section.