In Word it is called a Watermark and applies to each page created and covers the whole page.

In Excel it is easy to insert a picture but not possible to then confine that picture to a single cell. One can fudge a picture size to appear to 'fit' a cell but any change to cell height and width will not change the picture size at the same time because the picture is a separate object.

Also the picture cannot then be typed over the top of because the 'send to the back' is not an option.

The most that one can do is to set background colours for cells.