Favicon is standard across the internet. To get the latest update on this topic, check on the link below:
The logo that you upload in Tribe admin will be shown on the header of your portal. Whatever image you upload, the height of the image will be set to 40px. It is advised that you upload an square image with the minimum of 80px height to make sure there is a support for 2x image resolutions (e.g: Macbook Retina Displays)
The Portal Banner image in an admin refers to Open Graph Image Meta Tag Size (og:image).
That is an image preview when you share a link on Facebook or Twitter and other places where they support og meta tags.
Check this link for Ideal Open Graph Image Meta Tag Size: