During the course of my lurking, I've watched a couple of people generate some pretty awesome icons for use in HeroScribe for our HQ25 quest pack monsters - particularly the Chaos Dwarf Anderas put together:

One thing these icons all have in common is that they are exactly two colors: black and white.
I'm currently working on making a new Quest Pack this year (actually I'm just picking up where I left off in 2016, but you know how these things go). One of the Monster types I want to use for it is a Water Elemental, and since the only currently-existing icon for Water Elementals in the Scribe is just a wave, I thought it would be neat to re-use the art on the back of the Water Spell cards for a new one.
Thing is, I'm no artist; my talents lie more towards the realm of mathematics. So, having attempted a practice on the Fire card back first, this is unfortunately the best I've managed so far:


The first one looks pretty good, of course, but it's a grayscale rather than pure two-color black-and-white. And while the second image meets that criterion, the edges are a little too rough and don't quite capture enough of the original art.
Is there a relatively easy method you guys use when converting images to icons that I could learn? Any tips?