For PPT 2003 and probably all other versions... Once the template is on the
screen, Click on Format, Background. On the dropdown below "Background fill"
and above the radio button for "Omit background...." click on the dropdown
arrow. Click on Fill Effects and then the Picture tab.  Click on Select
Picture.... drill down to the picture you want as your background, click on
Insert, OK, the Apply to All. To save as a new template, click on File, Save
As, Change "Save As Type" to "Design Template.(POT) and then OK.  You have a
changed template with the new background.