Hi,

You can select the chart area and then use Chart  Tools > Format > Shape
Style > Shape Fill > Picture.
Or Format dialog, Fill > Picture.

If you want the logo complete below the chart you will need to embed a chart
within a blank chart sheet.
To create a blank chart sheet select an empty worksheet cell and press F11.
Insert the picture with this chart sheet and then use the Move Chart option
to also add your chart.
Position chart and logo as required.

Cheers
Andy