I know I can do it, as I have several times, but can’t find it today.
The help file says to go to settings/general, and drag the image into the banner box, but there is no banner box there if I go from the menu (where “settings” changes to “preferences” when I click it.
And the general setting on the left side says my theme (Basic Blue) does not support sitewide banners, which is wrong unless some update had eliminated that feature, which would really be discouraging.