The precomp in the examples in the sample projects is used to locate in time the stills so if when you animate the button to use Pix key-frame in RE:Flex Morph it is at the proper place in time. (eg still 1 at frame 1 and still 2 at frame 30 and still 3 at frame 35).
If the stills come from a sequential still capture rig you might also consider to perform this work with Twixtor using “Frame Number” parameter animation instead. Twixtor PRO and RE:Flex Morph with Auto-Align are similar except for how the material has to be prepared for the tool. For example it’s much more practical with RE:Flex to remove bad frames or animate photos (e.g. make them talk) from a small set of stills.
Pierre