Changing the logo is generally the same process as changing a regular image. The element is already included in the existing templates, forming part of the header (and often the footer) displayed on all pages, so there's no need to make individual changes on each subpage.
Learn how to change an image with the SiteDu.de builder
Do you want to add your logo at a custom location, but on every page of your website, for example in a second footer or a side menu, make sure you make use of the Shared Elements.
Learn how to make use of shared elements to display the same content on all pages.
Make sure to use the correct image format. While jpg files work, we highly recommend to use png logos with a transparent background. You can order such a logo (and also the removal of background) on Sitedu.de.