DB2 9.5 and above:

Is there any way to find out the list of tablespaces present in an ONLINE DB2 full db backup image?

if having access to the db you could do list history - the included list is present
if not - run db2 restore xx from xx taken at xx redirect generate script vvv
this will create a restore script and tablespace containers are also listed - this might help
db2ckbkp -t ... will also give ts details