How to resize images proportionally (aspect ratio) using generic handler in ASP.Net

I have images of big size. I want to resize them according to their width and height. If width is bigger than height then maximum width will be 650px and height will in proportion of width. If height is bigger than width then maximum height will be 650px and width will be in proportion of height.How i will resize images proportionally using handler in ASP.Net,C#.