You can define logo, favicon, and portal banner picture in the Admin Panel> Portal Settings> General Settings:
Logo in the main logo that will be displayed on the top-left of your community page.
Favicon is a small 16×16 pixel icon that appears at the top of a web browser. It serves as branding for your website and a convenient way for visitors to locate your page when they have multiple tabs open.
Portal Banner is the picture that gets displayed on top of the link widget when you share a link from your community in any other places, like Slack channel, Twitter, Fb, Telegram messenger, .... It does not get displayed in anywhere inside your community.