With radio waves. An antenna on the spacecraft sends a directed beam of radio waves towards Earth where another antenna receives it (typically one of the Deep Space Network). The data rate decreases with increasing distance, but it's enough to transmit images over billions of kilometers if you have enough time. New Horizons needed over a year to send back all the data it took during the Pluto fly-by. We still have contact to the Voyager spacecraft, 20 and 23 billion km away. Saturn is "only" about 1.5 billion km away.

There are some experiments with laser links for interplanetary communication, we might see these being used in the future.