If you have an iPhone 4S or later, you can add a filter which will appear live in the view while taking a picture.
In either the Photo or Square mode, tap the filters icon (three overlapping circles in the bottom right corner) to view nine options. Tap on one to enable it and then take the shot.
While a filter is enabled the filters icon becomes illuminated, tap this and change the filter back to None if you want to turn it off.
