Photo Scale is a single screen app that allows you to quickly rescale any image between zero and 200 percent of its original size. It won't handle all of your editing needs, but it has a handful of useful features that make scaling easy to do on the fly.
After downloading Photo Scale, you can open it and start using it to rescale images right away. Like most photo editing apps, you can either take a new photo or you can load an existing one from your photo library into the interface and work with it directly. After loading the image, you can drag and drop the slider to choose the aspect ratio you'd like to scale the image to. Then it's as easy as exporting the image or sharing it through the app to whichever e-mail, text message, or social medium you want to use. The process is fairly straightforward and the interface is one screen so the learning curve is fairly flat.
If you are looking for a full-featured photo editor, this is not it. It's not designed to be. However, if you want an app that will quickly rescale any images on your iPhone, then consider downloading the free Photo Scale app. It can rescale and export fast with any images already on your device.