Change aspect ratio of pictures
This guy works great for me on a Samsung 8.4" pro tab:

https://play.google.com/store/apps/details?id=jp.co.valsior.resizer

Has a long list of pre-set sizes and a select-your-own-size option, which like photoshop allows you to choose whether you want to maintain the aspect ratio or not.  You can choose file type and quality as well when saving.