The cleanest, most elegant and easiest to maintain long term solution is to use a category on UIImage. Define a custom method on UIImage, and then use a Find & Replace to replace all your imageNamed calls to your new method.
Don't use swizzling (it will introduce weird, undocumented bugs) and is very difficult to maintain or debug. (Also, if you swizzle, it will break in places where you do want to use the default implementation of imageNamed).
There's no need to subclass UIImage for what you're trying to do because you don't need an instance method.