On Windows Phone 7, I noticed the animatedimage control was not getting unloaded due to the timer not being stopped (forcing the control to stay in memory).
In the constructor add..
this.Unloaded += new RoutedEventHandler(AnimatedImage_Unloaded);
void AnimatedImage_Unloaded(object sender, RoutedEventArgs e)
That will stop the timer and force the control to be unloaded when it is no longer in use.