The cover photo for your organisation profile is uploaded in Org HQ > Details > Public Page > Edit logo & cover photo.
The cover photo for your cause listing is uploaded in Org HQ > Causes & Donations > Manage Causes > Edit.
GiveNow cover photos need to be rectangular and landscape. We recommend uploading an image that is 800 pixels wide x 310 pixels high.
See also: How do I upload my organisation logo?