Don't know if anyone has detailed how to make a .dds image for creating a new loading image for a route. Here's how I just did it.
1) get the DDS utilities package from http://developer.nvidia.com/legacy-texture-tools
2) create your image, jpg works ok. Must be 1024x512 or 1024x768. Seems like the latter gets truncated.
3) execute the nvdxt.exe utility, command line like: nvdxt -file <inputfilename> -output <outputfilename> -nmips 8 -dxt1c
4) use RWTools, go to Packaging, Package Route, Install New Route Image, and follow the instructions there.

