I think that when you sign up for Twitter you agree that photos you post can be retweeted. But being retweeted is different than being taken and posted elsewhere.
Just recently a photojournalist won a $1.2 million award against two large media companies who took his photo off of his twitter feed and distributed them without his permission.
Is Bobby Ryan going to mind you using his cute kitty picture on a blog? probably not, but technically, he could if he wanted to. It is always best to get permission.