Your arm should be slightly bent.It's hard to tell from most photos you see because the person'sbend in the elbowis parallel to the string and looks straight from the right side if you are gripping the bow properly. However, standing to the left or behind the same person you will see a slight bend.

Locking it in straight can result in torque on the bow as well as the possibility of the string smacking the snot out of your forearm on release.