Need a full-bleed image in your header? Use the wrapping and margin options on your image and you can get a full bleed image in your Google Doc header.
Watch how to add a Full-bleed Image to a Google Docs Header
In your Google Doc use the Insert menu then Header (or double-click the top of the page to add a header);
Use the Insert menu then Image and add the image you want (resize and crop as necessary);
Select the image and chose the Wrap text wrapping option and 0 margin in the image options box;
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.