Thumbnails cropping issue drives me nuts

My theme is based on a pretty extensive framework(but it’s free) and first I thought it has something to do with it. Looked at the php file and saw that is using only the default wordpress code <?php the_post_thumbnail('thumbnail'); ?> and not here is the problem. Whatewer thumbnail type I chose from the admin(thumbnail, medium, large) and specify the dimensions(570×200 for example) that one isn’t made by cropping the photos but scalled instead by the height(200px height and whatewer width). That stuff is driving me nuts as different thumbnail sizes look bad and I don’t know what could be the problem. If someone encountered this or could give me atlast a hint about it I’d much appreciate. ??