Hi,
is it true that I can turn off exclusivity on assignment files after 1 year? I've got a file accepted on 24 december 2010, but I can't find a way to turn off exclusivity.
Can you help me?

You need to dig deep into their site info to find the terms -
http://www.dreamstime.com/terms.php#contributors

"A Contributor may remove the exclusivity rights to an image with thirty (30) days prior written notice to Dreamstime.com."

You have to contact them via email to request a change to your image.

Thanks, I've written them an e-mail.