Before I found this great app, I tried several that claimed to "resize" but, instead, "cropped."
Finally, I found this one! Not only did the developers understand that "resizing" means "scaling" the entire photo without chopping off any of the sides, IN ADDITION, they understood the need to keep everything in proportion as it is being scaled.
It is exactly what is needed for quick resizing on the iPad. Simple, fast, easy.
Note: If you need to pick a specific interpolation method, you'll need an app with a fuller set of features, but for almost all of us, this app, is perfect.