This will add an & [Picture] code to the header. Simply click anywhere on the sheet and your watermark image will appear in the workbook. Now, if you want to put the watermark image at the centre ...

Select the "Insert Picture" button in the header or footer dialogue box. Browse to the location where you saved your graphic when you created it. Click it to insert it into Excel.

You can insert or add Header and Footer in Excel spreadsheet using this tutorial. Insert page number, current date and time, etc.