You can customize or change WordPress image sizes using this function: http://codex.wordpress.org/Function_Reference/add_image_size
<?php add_image_size( $name, $width, $height, $crop ); ?>
$crop parameter can be set to
false for proportional or
true for hard crop (it will hard crop from the center).
If you use this function on already existing images you will need to regenerate them.
If you want to manually crop individual images you can do this by default in the WordPress image editor, or use a plugin like http://wordpress.org/extend/plugins/tags/crop
If you are not comfortable with the above function you can use a plugin like http://wordpress.org/extend/plugins/simple-image-sizes/
To manually crop each image, click "edit" under the image in the pop-up image editor and you will see this.
To programmatically change the crop area, you will have to use a technique seen in brasofilo comment link.