how to dither to 16 bit color (and why is the AGS palet different?)

[EnterTheStory (aka tolworthy)]

Does anyone know of a program that can apply Floyd Steinberg dithering when reducing from 24 bit to 16 bit images? (Apart from Image Alchemy by Handmade Software, but that costs a hundred dollars.)

Also, does anyone know of an image editor that uses the same 16 bit palettes at AGS? I assumed that all 16 bit palettes are the same, but have noticed that if a 16 bit image (saved in Alchemy of Paint Shop Pro) is used as a 16 bit background in AGS 2.72, the colors change slightly.