To remove the motion effect from a button:
Click the thumbnail portion of the button (not the button label) twice, slowly, to select it.
In the Movie pane that appears (shown below), select the Still Image checkbox.
This stops the motion on the button.
To change the frame of the movie that shows on the button, drag the Movie slider until you see the image you want.
Click somewhere on the menu outside the button to close the Movie pane.