Stop / Start: button for starting and stopping the looping.
Step: click the left- and right-arrow for single frame stepping, backward and forward.
Rock / Loop: when clicked, the animation will turn to a "rocking" back-and-forth mode. Click again to set back to loop mode.
Speed: 3 frames per second by defult. Slide towards the right to increase
the looping rate.
Toggle: to toggle images on and off. As each frame is looped through, the
box corresponding to the image frame will change color. Blue box indicating the current frame.
Left-click on a box will hide that frame from showing in the looping (box becomes red); click
again will re-enable it. Right-clicks on a box, the loop will jump to and start looping from
that frame.