Use the correct single player image for the specific map part

[IncPlusPlus]

There are several map preview images for the different distinct parts of the single-player maps. Currently these are named out-of-order (in terms of chronology) and are picked randomly. It'd be nice if these could:

1. Be named in the correct chronological order
2. Have the right map part be used depending on what part of the map the player is on.

[IncPlusPlus]

I might just leave this as-is. It'll be a big pain in the ass to track down which is applicable at what time. Instead, leaving them cycling randomly kinda looks cool.