Remove image index from IM filenames

Fixes the problem of creating derivatives that look like this:  
![image](https://cloud.githubusercontent.com/assets/5470462/3043280/a0d7d2d0-e104-11e3-9501-06c9a10e009a.png)

 Basically, the existing code assumes that you always want tile 0 (or page 0 of a tiff).  You're better off trusting imageMagick.