Add a watermark in Excel

Select Insert >Text > Header & Footer.Insert Header and Footer
Tap on the header and on the Header & Footer Elements tab on the ribbon, click Picture.Insert Picture
Select one of the available options to insert your image. In the header, you'll see & [Picture].
Tap anywhere outside the header to see the watermark.Picture Inserted

Click the header in the spreadsheet.
On the Header & Footer Elements tab, click Format Picture.

Format Picture
In the Format Picture dialog box, select Picture tab > Color > select an option.

Format Picture