Thank you for letting us know where you are with regards to this skill.
On the background page click the down arrow at the bottom of the
"Background fill" frame and choose "Fill effects ..." The "Fill
Effects" tab opens with a "Select Picture ..." button available. The
default directory that it opens is probably "My Pictures." If your
picture is not save in that area, you can navigate to where it resides
in your file storage. Post back if you need further assistance.

If you download a template from microsoft it will automatically end up in the
right place.

From elsewhere the easiest method is to find & open the file just like a
normal presentation and then "save as" choose save as type "design template
*.pot" in the drop down menu (scroll down) .

It will save by default in the correct folder which is
documents & settings\your name\application data\microsoft\templates.

You may have to close and restart ppt before it appears in the design pane.