[img_fill/] resizes the source image proportionally to exactly cover the given width and height, cropping the image as necessary. If either dimension is omitted, the behaviour of this shortcode is identical to [img_fit/].
[img_fill output="{format}" width="{int}" height="{int}"]<source>[/img_fill]dimensions
[img_fill output="img" width="200" height="200"]atest.gif[/img_fill]
<img src="/wp-content/uploads/image-cache/1d2cf16c/atest_gif/fill_200_200.gif" width="200" height="200" alt="atest.gif" />
[img_fill output="img" height="200"]atest.gif[/img_fill]
<img src="/wp-content/uploads/image-cache/1d2cf16c/atest_gif/fill_0_200.gif" width="142" height="200" alt="atest.gif" />