Stay relatively simple and crop somewhat closer to the face than you'd like to get a cleaner image at 100x100. Really consider what kind of facial expressions your character makes; being able to express that in a few lines can make for a more clear end image.

That also applies to the 15 end images. The resources give a good general list but some characters list more towards one range of emotions than another, so you might want to sub in another kind of 'happy' or 'grumpy' or whatever works. I also gauge what icons I want to make off of recent threads, and keep making new ones every so often to refresh what I've got and purge the shit icons I made when the journal was still new and I didn't have as good a handle on the character.