The first image that included Earth's full disk (but less than half of that was illuminated) was taken from Lunar Orbiter 1 on August 23, 1966. Before that, images of Earth were from relatively low altitudes, so even were the sun directly behind the photographing satellite the view would show significantly less than half Earth's surface.
In late December of 1966 the Application Technology Satellite I (ATS-I) began sending the first images of Earth from GEO, including the first full-disk image of an essentially fully-illuminated Earth. Well, anyway, from GEO you only get ~42.4% of the entire surface, so ~84.9% of the satellite-facing hemisphere. ATS-1 also sent the first-ever movies of cloud movements on a global scale, and the "movement" of the illuminated part of Earth as it rotates.