To add your store logo to your stores header section do the following:
Click on the menu Configuration > Store Settings > General Settings:
You will now see a screen with "Edit Settings". Click on the "Image" tab".
Click on the image next to the "store logo" text. This will open a small popup with a "pencil icon".
Click on the "up arrow" icon. This will allow you to select a local image from your computer to upload. Once uploaded click the "save" button.