resize($width, $height, $constraint);
Returns a GD::Image object for the new, resized image. Original image is not modified. This lets you create multiple thumbnails of an image using the same Image::Resize object.

First two arguments are required, which define new image dimensions. By default resize() retains image proportions while resizing. This is always what you expect to happen. In case you don't care about retaining image proportions, pass 0 as the third argument to resize().