In the UK and some other countries radio amateurs are not allowed to use their licence from an airborne device, so we are restricted to using licence-free transmitters in some small, low-power bands where airborne use is permitted. The NTX2 is a popular choice for high altitude ballooning, and both the 434.075 and 434.650MHz variants are allowed. Maximum power is 10mW ERP which, surprisingly can reach over some very long distances (the record is over 800km) when paired with a good receiving aerial and a sensitive (amateur radio) receiver.
Baud rates are generally 50 baud but up to 1200 has been used albeit with a much reduced range. Images have been sent over these links, usually at 300 baud, using an encoding system that splits the original jpeg image into a number of self-contained 256-byte packets each of which includes FEC. I've used this on 3 flights so far (and am doing another one this week) and it works very well.
To the original poster, if you're planning on flying a balloon, I suggest (quite strongly actually) getting just a basic tracker running first, transmitting some simple telemetry containing the balloon's GPS position, before progressing further.