How do I crop only height?
You'll have to use custom GD methods I think. 

Have a look at this module : https://drupal.org/project/imagecache_actions

With the embed "Custom Actions" module you'll be able to use any gd method you want.

Add a custom filter to an image filter, and find the gd function to use.

In example, to get a B&amp;W picture, the php code to use is : 

return imagefilter($image-&gt;resource, IMG_FILTER_GRAYSCALE);