The purpose of this application is to re-scale icon images for iPhone & iPad apps (and Android too), because having to constantly re-scale images drove me nuts. It includes all the image sizes I could find (24 in total) and with the help of imgscalr  (http://imgscalr.com), it does a pretty good job of rescaling images. You obviously won't want to use all of the settings in one go, but you could break things down into separate lists.