If you want to reduce the intensity of the image, also change its opacity. After inserting it, right click on the image and select the Wrapping text option of Behind Text.With the Header area open Insert the image you want to be the watermark for the First Page into the First Page header area.From the contextual Design tab for Header & Footer Tools that opens, mark the Different First Page checkbox.Open the Header area of the document (do this from the Header menu on the Insert tab) and choose the Edit Header option.If you have a 2-page document and it is only 1-Section and you want a unique 'watermark' image on the first page and a different 'watermark' image on the second page, do the following.

Different First Page, which means a unique Header for the first page of a specific section.Primary Page, which is the odd numbered page.Each Section of a document has 3 potential Headers. I said all Headers and I said that on purpose. Word's built-in Insert Watermark function places the same image in all Headers of all Sections in the document. Watermarks are essentially images placed into the Header level of the Word document.

Sample watermark on all pages word