I don't think iStock accepts jpeg2000 files. They're a format that never took off. In previous replies to this question, I've likened them to Betamax, technically superior to VHS but it never reached the critical mass to become widely adopted. A/B comparisons of jpegs and jpeg2000 files don't show any real difference either. If we didn't have TIFFS or similar lossless formats maybe they would have been more popular.