Just thought I'd post this in case anyone wants to do the same on their blog.
By default, WP crops a thumbnail from the CENTER of the image. I am creating a Web Design gallery and generally the left hand side of my screenshots contain the important bits (logos etc). So i wanted my thumbnails to be cropped from the LEFT (not Center). Here is what i did (im using WP v2.7)
1) Go to wp-includes/media.php
2) Go to Line 288
3) Replace this:
$s_x = floor(($orig_w - $crop_w)/2);
//$s_x = floor(($orig_w - $crop_w)/2);
$s_x = 0;
(Commenting out the old code in case you want to center again later).
Using $s_x = 0; just means that it is cropped from the LEFT. Alternatively, you can use $s_x = $orig_w - $crop_w; to crop from RIGHT. Oh and you can follow the same methodology to crop from TOP or BOTTOM.
Hope it helps.