Check for missing image

When phantom fails to capture an image, it means there are an odd number of images in the folder which causes cropping issues such as #99.  The intention is to check for an even number of images in each folder, then add a standard image to ensure cropping doesn't fail.

![isvatk7](https://cloud.githubusercontent.com/assets/1492067/2549338/dbd98ab0-b673-11e3-943e-f3e11e65b224.gif)