Add a full-bleed header image

1. In your Google Doc use the Insert menu then Header (or double-click the top of the page to add a header);

2. Use the Insert menu then Image and add the image you want (resize and crop as necessary);

3. Select the image and chose the Wrap text wrapping option and 0 margin in the image options box;

4. Move and resize your image so it fills the width of the page.

You can follow the same steps if you want a full-bleed image in the footer.  If you only want to use this on the first page then you can use the Google Docs option to use a different header/footer on the first page. If you don't want to use a header and footer at all then you can get the same effect by setting your margins to 0 but this will affect every page and no pages will have margins.