This is a very common and simple image manipulation problem solved by most image editors, as far as I'm aware. So the better question is do you need to automate it, do it programatically, or batch-process, or something else complicated? Otherwise many applications should do what you want with a simple Resize function. For example XnView handles this easily. Just open the resize dialog, make sure it's set to preserve aspect ratio and *not* preserve print size (which would change DPI as you changed resolution), and then enter your new Y value, the X will adjust automatically. Note also that XnView has a batch processing function and I think there is even a commandline version and/or a library that you could access in code to do things programatically.