Awwwww, sooooo cute! I love the detail on their hair and eyelashes, and the water looks great. Oh, and their eyes are so adorable! AND I love Derpy in the background! Plus I love PinkieShy. Overall, I just love this picture.
These lines you use to add structure to her mane and tail are not as clearly as they are in the lightended parts. At least it's not so easy to find out where her mane ends and her tail starts at first view. Somehow the shadowed parts look like they are part of her mane and through the blurry effect it's connected to her tail as well.