Are you using resample.exe...it looks like it.
For seasonal textures, you have to have bmps that appears like you want for each season. So you start with source image for sp, fa, wi, hw, and lm (night).
I have my own preference, other ways exist. I like to have a folder for each, make an inf file for each, with the command withseasons=1. and I just run resample.exe on each one from a batch file with the following commands
resample filename.inf
rename ???????????????su ???????????????wi
Of course I show the winter bat file here.
Make sure the summer run of resample was also done with "withseasons=1" command in place.
Have fun!
Bob