I don't know anything about reopening, if you've previously closed your account, you probably need to start again from scratch, though someone might know more.
If someone wants to start from scratch, they'd go to the iStockphoto home page, and click on the Sell Stock link at the very bottom of the page, which at the moment takes you to a page with this message:
Thank you for visiting our Work With Us website. We�re currently performing maintenance and our engineers are working hard to get Work With Us back online. Thank you for your patience and please check back soon.

So you click on a link called "Sell Stock" and land on a page which says "Work With Us". Do they know nothing of the principles of web usability? 

i hope you've thought long and hard about this decision!