How to handle different uploaded dimensions pictures and resize them to standard size(dimensions) without losing quality?

For my Iphone and Android App. I allow users to upload images(which can be of different dimensions). These images are later displayed in the app(TableView). How 开发者_Python百科can i assure that the images that are being uploaded by different users will have the same dimensions and quality in the Mobile App?

I've been using this Simle Image Class for a long time. It works great (except bmp).