Don't forget to activate the *Auto Refresh* option, put the *Offset* at 0 if you want to display the first picture, keyframe the *Offset*:

[![enter image description here][1]][1]

Move forwards in your *Timeline*. To switch to the second picture, put the *Offset* at 1 and keyframe. Choose a *Constant* interpolation between the keyframes:

[![enter image description here][2]][2]


  [1]: https://i.sstatic.net/331oB.jpg
  [2]: https://i.sstatic.net/QwOVK.jpg