They've ripped you off, right?

But there's nothing you can do about it. If you upload to Postimage and then go through and edit in the 'hotlink for forums' codes to put the pics back you can overcome this.

A lot of work, but there's really no alternative. Though Postimage is not the only free service, and it might also be worth setting up your own webspace. You might have some in your deal with your ISP, and you can certainly buy it from many places.