ImageKill removes images which match certain size criteria. The
idea is that many advertisement banners have a certain
width/height ratio by which they can be recognized. This makes it
possible to remove ads which don't give themselves away by their
name.

The filter works by examining the source URL for the image,
looking for "width" and "height" attributes. This means that
loading an image can only be avoided if its source URL contains
those attributes (which is often the case). The advantage on the
other hand is that we don't have to load the image just to find
out its size.

To avoid removing navigation bars and other images that you want
to see, even if they match the evil width/height ratio, ImageKill
can prevent images from being removed in two cases:

* The image source tag contains a specified string/regexp;

* The source tag contains the "usemap" attribute, i. e. it's an
  image map.

This filter has the following configurable preferences:

* ImageKill.minheight

  Only remove images which are at least n pixels high.

* ImageKill.minwidth

  Only remove images which are at least n pixels wide.

* ImageKill.ratio

  Remove images which are more than n times as wide as they are
  high (in addition, they must match the minimum sizes given
  above).

* ImageKill.exclude

  Don't remove images that match the given string/regexp.

* ImageKill.keepmaps

  Don't remove images that contain the "usemap" attribute in
  their source URL (image maps).
