There are lots of ways, one possible way being an indication in the beacon package via APRS or WSPR that a new image is available. "SSTV" is a protocol for send

There are lots of ways, one possible way being an indication in the beacon package via APRS or WSPR that a new image is available. "SSTV" is a protocol for sending "Slow Scan TeleVision" images over the air on 10 meters which is 28 MHz which has decent propagation. Generally such transmissions are energy intensive and might blip on then blip off given a solar charged power source. Most Amateur balloons just tell you where they happen to be at the moment via GPS co-ordinates.