[Andrés]

Maybe it's because it looks for default english se4 path name and can't find it, but it always seems to start in the root directory of my hard drive. No matter whare the builder is located. And I have to browse all the way to the races.

In most cases 1st choice for troopinfantry should be race_portrait/pop_portrait.

Your minimun requirements look too high.
I think you only need the portrait battlecruiser to choose in the add new race window. You'd need at least 1 mini to copy, but not necesarily the battlecruiser. Maybe if battlecruiser is not abailable the program should attempt to copy another pic there too.
BigExplosion and Shields are not indispensable either, there are a number of races that are considered complete and do not include custom shield or explosion, and by the way generic pics tend to match different shipsets better than ships here.

What about the idea of an easily editable .txt file defining ship sizes and copy chains.

This is probably beyond your programming capabilities, but maybe you can find some pre-made library to resample bitmaps (note the difference between resample and just resize). You could make it copy and resample race portrait to create pop pictures.