I don't know what values you want to rotate between, so I can't give you code - and if I did how would you learn?
@Nux gave you a big hint above, use image_angle.
Draw a sprite to the screen and draw the "image_angle" (or output to console) - then add/subract 1 from the image_angle every time you press a key (left/right arrow for example).
When you have the 2 values for the left and right rotation that you think look good, note them down.
Then in your code, set these as 2 variables minRot / maxRot, and another variable to store the current rotation direction.
In the step event of the talking head object you can then add/subtract from the image_angle until it = minRot or maxRot (depending which way it's rotating), and when it hits the min/max value reverse the direction of the rotation by switching up the value you add to image angle (+1 or -1) so it will rotate the other way.