I think, in a piece like this, dither is a poor choice. Dithering suggests a rough texture(most of hte time), while a frog like you've chosen has a very smooth, wet texture. To emphaise this wetness, I kept my middle values and shadows close in value and kept their lightness as far away from 100 as I could. Then I placed a few choice specular whites.
Here's a couple of ideas that would help improve the piece followed by an edit.
Hue-shift. In your original, you made each one of your shades by darkening the original colour. You don't want to do this because it's monotonous-looking and not at all how things tend to work in real life. The topic on light a huge one that I've barely got a grip on myself, but a general rule is that your lighted area should shift their hues towards yellow and your shadows towards purple/blue. The reason for this is in real life the highlights tend towards yellow because of the sun's light and purple because of reflected blue from the atmosphere.
Saturation. Almsot all your colours have 100% saturation. I'm not a big fan of this, because it can kind of make your eyes bleed, and you don't need to use taht kind of saturation to make something appear bright. It's especially bad on shadows because the saturation makes your shadows pop out which flattens your image.
One thing to keep in mind, just because something you're drawing is coloured black or white doesn't mean you should go and use a super bright white or black to represent that. Our eyes can be very tricky when discerning the colour of something. A sheet of paper on your desk might appear to be white, but take a picture with a camera and it the picture shows it as an almost neon orange, a result of the a incandescent light nearby. Our perception of the colour of something is strongly influenced by the colours around that something.
Angle of the source image. I did a google search and found your reference and I noticed that the head on hte source was pointed more towards our right. The lightsource on the reference was knda hard to figure out so I fudged things a little. I think I could've tried to get the shape of the head better in my edit. It looks like there's a depressed ridge area between the eyes and nose, but I didn't show this well. I'm not sure why I made the black part of hte eyes so big. It ended up like that by accident and I just liked it. Blame anime, I guess.
Hope this helps.
Also, do remember to post a link to any reference images in your future posts. It's helpful to us when critiquing to see the reference for comparison. More importantly, though, it's essential to credit your source(s).
The last thing you want to do is accidentally violate rule 2EDIT: As a further note, just in case that was put too bluntly, it's very obvious that this is all your own work and I don't mean to imply anything. It's pretty common thing on art boards to post your references in small part due to a tiny, tiny minority of people trying to pass off others work as their own, but mostly because it's helpful when critiquing to be working with the same tools as the artist.
