I think you could use colors with more contrast just to make the sprite more interesting. Her arms looks somewhat limp (not an native speaker, so couldn't find an better word), working around a little bit of anatomy could be great; Like her waist could be a little bit bigger so she don't look like so "straight".
I like the idea you had to make dithering for the scales, but i don't really think you need to use it, at least not to this extent.
I made an edit and even wrote an text, but the images aren't appearing for some reason, but you could take a peek at some pixelart basic tutorials to make round objects and such. And change her tail to look more natural.
Here's the medusa sprite from SOTN, i really like this sprite... You can use it to study a little, studying pixelart using sprites from some games is pretty good