As it turns out 4images doesn't need the name of a specific program, as the config implies, it needs the path to the binaries, got that figured out.
Now I'm getting zero byte thumbnails. Other threads have shown that this is most likely due to a version of Netpbm that isn't right for the OS. All of the links I've seen to pre-compiled versions of netpbm either don't work any more or don't have the package listed. Can anyone point me to a current link?
Thanks!
SR