cannot use a bagfile to transport images

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Hi

I am working with a bag file that publishes these topics :

/clock
/newcollege/camera/image/compressed
/newcollege/odom
/rosout
/rosout_agg

I want to subscribe to a topic to call a callback called image_callback in python. I do :

sub=rospy.Subscriber('/newcollege/camera/image',Image,image_callback,pub_vt)

But I cannot do that.

When I get images from Turtlebot, it works. What could I do ?