Freak Booth is a free app that allows you to apply distorting filters on portrait-style images on your iPhone. The result is rarely much more than a pinch or expansion of the face due to the limited number of filters; and while the app is generally very easy to use, the number of options never overcomes the lack of depth in how they are used.
When you open Freak Booth for the first time, you can choose or take an image with the camera and then load it into the app. From there, you can apply the targeting reticule to the screen and then choose the style of filter you'll use -- pinch, expand, or twist. Tap the "Apply" button to apply the filter and it will be applied to one degree. You can move the target, reapply the filter, or change it to another filter from this screen, all of which can be done very quickly. When done, you can export to the camera roll or share on Facebook or Twitter. The interface is a bit cramped and there are few options to choose from, but everything works generally as advertised. The problem is that many of the tweaks feel like variations of the same theme; and without other editing options, it can lose its appeal after a while.
Freak Booth does what it promises and distorts photos with a few quick taps, but it doesn't offer nearly enough ways in which to make these distortions, so you may find it is not as deep or as effective an app as you would like for your purposes, especially since a lot of free editing apps will also offer those filters.