In Drupal 8, the default logo needs to be a SVG file. Add this to your theme like this: themes/custom/mytheme/logo.svg. Drupal will look here for the default logo. 

Under /admin/appearance/settings/mytheme, make sure "Use the logo supplied by the theme" is checked. 

Next, you can add the "Site branding" block to a region and the SVG logo will be shown there.