The Site Logo field (Under Admin > Site) is used for your header logo. The logo will be capped at a width of 340px and will decrease dynamically to a max of 140px in width when the site is viewed on smaller devices.
People > Church > "Default Church" - Mobile Logo
If this field is populated with an image this will be used for smaller devices in place of the site logo.
If you'd like to have a smaller logo icon display on the mobile view, you can add this separate image file to People > Church > select default church > "Mobile Logo" field. If this field is populated with an image, this will be used for smaller devices in place of the site logo.
Note: if you have multiple church records setup for your campuses, the record with the "default church" group assigned is the one where this logo file needs to be added.