As decided during our sprint, there should be a GUI for picking up a favorite image in the stack, perferrably something with decent GUI for comparing images side-by-side.
A quick & dirty hack (no GUI, just the possibility to mark an image as the first one from the ThumbnailView) got added in r926246.
To me this looks like fixed. The context menu -> set as first image in stack works. What more would you want to have?