Favicon is not created by uploading a site image. The customer will need to create a favicon.ico external to the builder, then upload it to their files.
Favicon must be 16x16 pixels, and added to Files Uploaded folder as Favicon.ico before using the following instructions.
How to add Favicon:
1) Under "Site" menu on left side of builder
2) Select Pages
3) Double click Page or background
4) Click "Advanced" Tab on right side of builder
5) Within Head section use the following code to point to that uploaded favicon.ico file. For example, if the file was placed in their "uploaded" folder
<link rel="shortcut icon" href="uploaded/favicon.ico"/>
Make sure to publish changes.