Those BMPs are actually used as placeholders in the manual render of the level select screen. The game only uses the ice variant, which is never displayed unless a level does not have a bmp specified for it.
As for the priorities, there are a bunch that are unused. Unfortunately only two of them work and of those two only one is useful. Furthermore, the construction priority that is active by default in the vanilla game is actually entirely broken. It doesn't work lol. In my mod I just replaced construction with the BuildPath priority. See this topic for more information. See this topic for more info.
The other usable priority is for handling the construction barriers, ftr.
EDIT: aiden's post also links to a good topic on the priorities. Most of the information prior to my post in that thread is unfortunately a bit embellished though...