Well, it took a lot of trial and error, but I was finally able to get the GIMP script to work with my input maps (protip for others who are technologically illiterate like I am: use .png files rather than jpegs so that oceans are colored as alpha rather than white). I made a few tweaks to the maps seen above and came out with the following generated climates, and the script spat out the following map:

Climates_gen.jpg

Overall I think it looks quite good, but comparing the Earth climates generated with the script to the "real world" climates it seems there are a few systematic errors it makes. In particular, it seems that Cfc climates are highly overstated, to the extent that most of what's deemed as Cfc is actually better listed as Cfb. It also tends to "under-generate" Am climates, so I changed some areas of Aw and Af to Am so that there would be more of a transition zone as seems to happen on Earth. I also changed all As climates to Aw, since there's not really a practical difference between the two when it comes to actual biomes. The edited map is below:

Climates_edited.jpg

I'm fairly happy with this map as it stands, although I'm sure I'll continue to make some small adjustments as I look at it more closely. However, if anything looks grievously wrong with it, comments and criticisms are welcome!

Now to move on to adding more details to this world...