Has you activated you account with them ?
Then somehow managed to remove the cokkie they would have set you (think this is how they do it)
If so then this would just upload the image to a sort of temp imageshack account, that upon closing your browser and trying again you will end up in a different temp account.
If you signed up, you'll need to re-click the link in the email they would have sent you, or im sure theres somewhere on the site to reactivate your account.
Ive had to do it a few times myself, as i kept loosing the settings for some reason
