Are you talking about an actual ePub ebook that you create from a Pages Word Processing document, or just a PDF you create from a Pages document?
If you make the image floating or in the background, you can move it around without being hindered by the margins or other text. To make your object floating or in the background, open the Inspector window (with the blue "i" icon in the toolbar), then click on "Wrap inspector" (third button from the left), and choose your Object placement.
However, converting to ePub removes floating objects, so if you want that, you should edit your eBook (using sigil for example) to add the first page back in. This is not too hard if you know some HTML.