The resize_image function allows you to resize a GIF, JPEG or PNG file to any dimension you wish and put an optional black border around the image. Here's an explanation of the arguements:
$image_path: The complete path to the image to be resized
$max_width: Maximum width of the resized image. Leave 0 if you want to specify only the height and have the width auto-scale.
$max_height: Maximum height of the resized image. Leave 0 if you want to specify only the width and have the height auto-scale.
$file_prefix: This will prepend a string onto the resized image filename. If you want to create a thumbnail image and keep the original, set $file_prefix = 'thumb'.
$dir: Allows you to save the resized image to a sub directory under the $image_path directory. If you want to save thumbnail images to images/thumbs, set $dir = 'thumbs'.
$border: boolean, 1 = border, 0 = no border
$mime: Image mime type. (image/jpeg, image/gif, image/png)