Roberocity.ImageHandler is a project mainly written in C#, it's free.
Simple C# Library for dynamically resizing images on the server
Setup the handler to handle all image types. Then you can pass the query string params of Width and Height to dynamically resize the images.
Does have a dependency on Roberocity.Storage, but that can easily be eliminated.
It's yours. Use it freely. Modify at will. Make it better.