If you do a Google search for "german flag image" you will get several.
Assuming the image is not copyright protected, right click the image and
click "Save Picture As". Once saved to your hard drive, open PowerPoint,
click "View", "Master", "Slide Master", click the "Format" menu, select
"Background", click the dropdown and select "Fill Effects", click the
"Picture" TAB, click "Select Picture" button, browse to and select your
image (preferably PNG or JPG). Your flag will now be the background of
every slide you create. Keep in mind the color differences will require
text colors that can support it. I would NEVER do this because of these
color differences, but...