Do you mean the image background, on which there is text, as follows:
In the Excel toolbar selected in the format "-" sheet-"background")
Select the picture adocumented
But this picture to many to fill the entire sheet background, and literal is not prominent.
If you use WORD to do much better results can be achieved, the background color of the watermark, the selection will be lighter, so the text will be more prominent. Successfulpractice is the same as the above mentioned EXCEL, but background there would be one picture.
The toolbar selected in the format "-" sheet-"background") the print watermark
If you only can do, or you can download free software, first put the picture into a watermark (watermark), and then inserted into the document.
Http://www.watermarker.com