Either of the following:
1. Open PowerPoint
2. Click "View", "Master", "Slide Master"
3. Click "Format", select "Background"
4. Click the dropdown and select "Fill Effects"
5. Click the "Picture" TAB and click "Select Picture"
6. Browse to and select your JPEG
7. Click the "Close Master View" icon on the toolbar
8. Click the "File" menu and select "Save as"
9. Click the "save as type" dropdown and select "Design Template (*.pot)"
or
1. Open PowerPoint
2. Click "View", "Master", "Slide Master"
3. Click "Insert", "Picture", "From File"
4. Browse to and select your JPEG
5. Position/Size it to fit your slide
6. Click the "Close Master View" icon on the toolbar
7. Click the "File" menu and select "Save as"
8. Click the "save as type" dropdown and select "Design Template (*.pot)"