You can create a class of fixed
position: fixed !important;
and apply that to any element for it to be fixed.
So you could put an image at the bottom of the layer order and give it the same class.
And then create other elements that will scroll over the top of it.
To change the images similar to your other example I'm thinking some code to change the class (reset) so it's no longer fixed and scrolls along with the other images until you reach the next image you want fixed and so on
Edit* I have a feeling the update which is on it's way should make this task a lot easier