>>1913437source image is already rendered (and better resolution)
http://gelbooru.com/index.php?page=post&s=view&id=1861710>>1913444>>19134211. use GIMP
2. layer->transparency->color-to-alpha with white as the target to remove all white from image to create a partially transparent monstrosity
3. make a new solid fill black layer and drag it below the transparent layer.
4. make a new transparent layer between then and paint white under all the non-background parts (character, fire highlights, logs). I kept a copy of the original layer and used selection tools to make this part easier.
5. the result is that it looks like the original image everywhere you painted white, and everywhere you didn't is now fading to transparency.
in general, doing any sort of complex masking is going to be way easier if you can use color-to-alpha. the standard case is isolating an object from a white background–usually you'd select the background and cut it, but then you cut off parts of the linework as well, or else you miss parts and get white fringes. with CTA you can just select the background, grow the selection by a px or 2 (so it overlaps the linework) and then use color to alpha on the selected region. it removes all the white while leaving the linework still intact and opaque.
pic attached is after step 2, look at it on a non-white background to see the effect