1. What is a favicon?
Favicon is an abbreviation of favorites icon, as known as website icon/page icon/urlicon. Favicon is an icon that relates to a website or a web page. Favicon.ico is usually used as a thumbnail of website logo, which will show in a browser's address bar or a tag. All major browsers support favicons.
2. Upload a favicon to Zsite
Go to "UI"->"Interface"->"Logo". Click "Upload favicon" to upload. Please note that the favicon has to be.ico file.
3. .ico converter
A lot of .ico converters are available online. Upload a picture and it will generate an image in .ico format for you. Here are some favicon converters