That sounds like you've used up all your space. You can delete some of your pictures (User CP > Attachments) and gain back some space.... or you can use an online hosting site to store them, then paste the 'img' code in the thread (what I do).
"What, exactly, is the internet? Basically it is a global network exchanging digitized data in such a way that any computer, anywhere, that is equipped with a device called a 'modem', can make a noise like a duck choking on a kazoo." - Dave Barry