You can use the slider bar at the top to crop to a shorter subset of images to loop. Once the images have loaded, stop the animation and drag the slider to the start of where you'd like to loop. Shift-Click the blue button to set the starting point of the loop. Then drag the slider to your desired end point and shift click again. Alt-click on the slider bar (option click on a mac) will reset to the whole loop

Mobile devices - 2 finger pinch to zoom/pan