In this video I demonstrate how to put a picture in the background of an individual cell. Technically this is not possible, but there is a workaround.

If you have tried to use 'Send to Back', you may find the option is greyed out which means it is not available. 'Send to Back' will only place a picture behind another picture, it will not place a picture behind text within a cell.

The workaround is to place a rectangle in a cell, fill it with the image and then type into the shape. The text will appear in front of the image. It's not a perfect solution, but it's the best one I have found.