Once docked with the Agena vehicle (whose L-band antenna is in the left foreground), the astronauts used its propulsion system to increase its apogee to 740 nautical miles (1,370 km), the highest Earth orbit ever reached by a manned spacecraft. 

This record stands as of 2019, even though men have achieved greater distances from Earth by flying to the Moon in the Apollo program. The maximum operational altitude of the Space Shuttle was lower, at 600 miles (970 km). 

This photograph taken looking west with the Hasselblad Super Wide camera and its 38mm lens shows the curvature of the Earth over the Indian Ocean and Sumatra-Java under partial cloud cover from an altitude of 530 nautical miles.

From the mission transcript during the highest orbit reached by the crew (orbit 26):

041:02:40 Conrad:

I’ll tell you, it’s GO up here and the world’s round.

041:02:43 Capcom (Mission Control):

You have a good view?

041:02:45 Conrad:

I mean it’s spectacular!

041:02:47 Gordon:

Bill, it’s really fantastic: You wouldn’t believe it!

041:02:50 Conrad:

I tried to yippee out the left window and..

041:02:57 Conrad:

- - ... under our nose and you’re out the right window.

041:02:59 Capcom:

Get some pictures out the right window, not the left.

041:03:02 Conrad:

Actually, we’re taking them all out the right window except the [garble] camera.

041:03:08 Capcom:

Roger.