We've occasionally used iStockphoto.com to provide photo's for presentations and advertising work. We've just received notice from them that they are "expiring" our credit because we've not used the account for a while. We regard that as theft. We don't care what their terms and conditions say, it is plain simple corporate theft. No doubt they'd take a dim view of anyone misusing the copyright material, but they have no problem with their own theft tactics.
If they do not want to continue to do business with us, they should just refund the outstanding credit.
They've now been added to our email blacklist and will no longer be recommended.