The logo should not exceed 250 x 100 pixels.
The image file format must be JPG, GIF, or PNG.
The image can have a transparent area or areas with alpha set to 0.
Uploading a Logo
You can view and manage your logo in Storefront › Logo.
To upload a logo, select Upload a custom image to use as your logo, then click Select an image from your computer or drag an image into the box.
Use the Logo Size setting to adjust how your logo is sized.
One added you will see a preview of your logo: