If you're building a seed site, then use a set of generic logos as placeholders. You will replace these in each site you build later. You can easily create these Seed branding images for yourself or you can use mine. If you want to use the ones I created, just right-click and save each image.
You will need the following at a minimum :
Upload each of these (or your own Seed site logos and images) to the media library. Make sure you also specify an alt tag for each image such as "Seed Site Logo".