For cropping a post thumbnail image to 200×300 pixels:
<?php
the_post();
img(200, 300);
?>
This will verify the existence of post thumbnail, crop the image, save it in uploads folder, and generate an image tag.
To verify the existence of a post thumbnail, you can use has_img()
<?php if (has_img()): ?>
<figure>
<?php img(200, 300) ?>
<figcaption>Some text</figcaption>
</figure>
<?php endif ?>
To crop images that are not post thumbnails, you can use crop($url, $size)
<?php
$cropped_url= crop( get_bloginfo('url') . '/wp-content/uploads/image.jpg', array(200, 300) );
?>
<img src="<?php echo $cropped_url ?>">
This plugin is available for download to be used on your WordPress self-hosted installation.