Excel allows you to add background colors to cells, including gradient fills and patterns, so you might expect that it would be possible to use an image as the background of a cell.
It isn’t possible to do this in the current version of Excel; images can be added, but they are ‘free floating’ and not tied to specific cells.
Adding images to an Excel workbook
You can insert a ‘free floating’ image by clicking: Insert > Illustrations > Pictures
Perhaps Microsoft will add the ability to use pictures as cell backgrounds in a future version of Excel, but for the moment this is the only way to insert a picture into a workbook.