Make image/color dropdown arrows match block color

# Before

![image](https://cloud.githubusercontent.com/assets/206973/3263299/4cea379e-f270-11e3-80d2-f38306c9350c.png)

# After

![image](https://cloud.githubusercontent.com/assets/206973/3263301/55156772-f270-11e3-89fc-c36318ee20a8.png)

![image](https://cloud.githubusercontent.com/assets/206973/3263307/5fa3ae74-f270-11e3-9afb-91c556608241.png)