Hi and welcome to the forums.
I chose this piece as we both seem to find it to be one of the strongest. It's got a good foundation, which makes it relatively painless for me to improve upon. The stongest part of the sprite is the balance of the shape and silhouette. There's a smooth, almost whimsical flow to the post, and the arrow has nice round and bumpy edges, which add to it's character. I also noted the weaknesses, such as the lack of contrast, saturation, and the large amount of colors used. These issues are addressed in the commentary.
Now time for the play by play.
1. First things first, I get rid of the RETINAL BURNING MAGENTA. Although I was unable to show it here, I am constantly changing the background color as I work. It lets me see how the sprite will feel with different hues and values. I'll go with everything from very dark to near white, with both heavily saturated and extremely pale colors. Constantly testing different surrounding colors helps to balance out the sprite's color composition and spot errors in contrast and readability.
2. I use black and dark brown to strengthen the outlines. You'll often see this used, where the outline gets slightly lighter in areas where the light hits it. The difference is subtle but it makes the sprite softer than if it had been made with completely black lines. I'm focusing primarily on blocking out the forms here. Also, note the huge discrepancy in color saturation and contrast. This and all the other sprites suffer from a washed out palette. Make some bolder color choices to really get the images to pop.
3. Here I add a shade to insert some grain into the wood. The color isn't a simple darker shade of brown, there is some blue thrown in there. In real life, all natural shadows have some blue in them due to the sky.
4. I add a buffer, or a shade between two existing shades to smooth out the transition from the lightest brown to the darker tan. This also allows me to pull out some more highlights in places.
5. I address the leaves. Again, saturation and contrast are the keys here. There's also a good amount of blue in the shadows.
6. I dip into the darkest black and use it to start adding more detail to the wood, and to add anti-aliasing to smooth the outlines.
7. Now I go back to the dark tan and use it to smooth out areas and add more form to the sprite.
8. The final tweak step is the longest and least organized, but probably the most important. I go in and fix anything that's bugging me. I was unhappy with how straight the top of the arrow was looking, and the largest changes to the sprite are here. It required changing both the position of the bolt and the flow of the grain. It was a bit more work, but worth the effort.
Something I didn't touch on was making every color count. In a lot of these, you have too many shades that are so similar that they really aren't making any difference. A lot of it may be due in part to Photoshop's tendency to smooth pixels whenever possible.
So there you go. I may have a hankering to have a go at something else after resting. Hope this helps.