Logo (Header): You can upload your own logo image at your Dashboard –> Appearance –> Customize –> Header settings (The default logo will be automatically replaced).

The supported image format is PNG or JPG/JPEG. The recommended image size is about 350px to 500px wide, because the with of your blog on Desktop is about 1000px. If you create the logo image by your-self, be sure you removed the margin or white space around the logo text before export it.