Hi,
James has the easiest solution. However ever if you want more logos as individual files you should look at the links James provided to MoodleDocs.
I would defiantely recommend making a duplicate of a certificate (using a different name) to make sure that works. Then as James suggested changing one of border images for one which also contains the images. Alternatively you can add your own images, or replace the stock images with your own. For example, I replaced one of the background image with a corporate seal and the "Team Work" image with a logo. If you want to have a different file name and a different postion, read MoodleDocs carefully.
Enjoy,
Chris