A favicon is the icon that will appear in your browser tab.

This will need to be a jpg, png or ico file and be of equal height and width. (*must not exceed 100×100 pixels).

To add/edit a favicon click SETTINGS > STORE SETTINGS from the left hand navigation.

On the Store Settings page select the [Appearance] tab and then select the [Images] drop down.

You can then either click on the [Favicon] image box and select your favicon, or drag and drop your image into the box. 

After you save the changes, the favicon will take effect.