The template sky file is in the \Support\rFactor folder in your BobsTrackBuilder folder. To modify mas files you'll need Mas Explorer from rfactorcentral (Addons > Mod Tools > Mas Explorer).
Because the images are .dds files, you might find this useful: http://developer.nvidia.com/object/dds_thumbnail_viewer.html
While I'm here I may as well add that you can use this to allow Photoshop and PSP to work with .dds files:
http://developer.nvidia.com/object/photoshop_dds_plugins.html
Or you can use GIMP (http://www.gimp.org/), which is free, along with the GIMP dds plugin (http://registry.gimp.org/node/70)