I have a header image which is exactly 7 inches in width. I made it 7 inches because I saw that the default latex margins were 1.25 inches on both sides, and I subtracted that from the default 8.25 width of the document. I was able to easily add this image in its full width on the first page using
But obviously this doesn't apply to every page. Unfortunately, other similar questions involved people wanting the image in the upper left, or the upper right. I just want mine at the very top of each page, and with the same margins as the page.