Never done one of these before, so if I don't address certain things as well as I should have, let me know.
Anyways, I only have a couple concerns regarding the image, mostly to do with the physical characteristics of the image.
1. Either the cow is quite small or the girl is quite large, considering the scale of the two. I'm not sure if this is something you could clarify in your description (perhaps it being a calf, not fully grown, for example).
2. The girl's right leg seems to be too small in my opinion. It doesn't seem bent enough to equate to the left leg if it were fully extended. Her left leg does seem to be in proportion to her overall body, however, so that's good.
3. The cow's second leg (from L to R) does not look exactly right, but that's a small complaint.
Besides those things, I like the girl's outfit and her sort of nonchalance in regard to riding a cow. It shows personality and direction in your work.