You can upload a logo for your shop, that is then displayed on every subpage of your shop.
To upload or change your logo, select in the administration area in the main menu Settings and then General. On this page you can edit the logo under Shop details. Save the changes.
The logo should have a size of at least 200 x 200 pixels.
The size in which the logo will be displayed in your shop depends on the design theme you have selected.