I have no affiliation with the company, just for disclosure...I needed a resizer I could use outside of my main desktop where I have advanced editors installed - I wanted something I could put on my netbook or work computers that was quick and easy and free - I found FastStone's resizer, and have used it for many years. It's very simple in that you can batch resize (up or down), choose from many different types of resizing algorithms or let it choose the best - plus you can even do some basic batch editing such as adding sharpening to all the resized photos which sometimes is needed when resizing loses a little sharpness. And you can also batch rename...it's pretty quick in my experience. If you want to try:
http://www.faststone.org/FSResizerDetail.htm
It's freeware for home users. It will allow you to save all the resized photos as a separate and new file, so you don't damage your originals. If you're trying to resize them right on the phone and not using a computer, I'm not sure about apps that do this - I think Faststone is just to be used off a computer/laptop type device.