Select the image, then click on Draw, select Order and send to back

In addition to Michael's advice, you can set the background of the text box
to your picture by using Format--> text box. One disadvantage to this is
that the picture may not compress when you do compress all pictures in your
presentation, so it may make your file size somewhat bigger than it would be
other wise.

If you do keep the text box and the picture together, then be sure to group
the two objects so that they don't get separated.