I'm making (what I think is) a wicked awesome eye set, and I wanted to make some of them defaults.. How do I do that?

I have Wizards and Paint.Net

Find an existing set of eye defaults. Export the textures from the finished eyes you want to use as defaults in SimPE using the .png format - just right-click on the texture and choose Export. Use the nVidia DDS Utilities to Build DXT in the existing eye default set and replace each eye with your .png textures in DXT3 or higher. If you're confused as to all this, look at the tutorials on skin defaults in the tutorial section here (Body Shop Modding, where I moved your thread) where there's links to the DDS Utilities and instructions on how to use 'em.