This plugin has been written to provide various ways to return an *image thumb* either as a URL or IMG tag. There are four functions.
Many WordPress themes, are heavily dependant on "feature images" to use as thumbnails to make them look good. This is great except it adds a lot of extra work setting up these thumb references, but equally if you are in a hurry and forget you add the thumb then the look of the theme breaks.
So here is Tui's Find Thumb plugin. It is very easy to use. It's purpose is to replace the Image function currently used in your theme with one that is lot more clever. With these new functions available you can do everything from a randomly select an image, or return a URL or IMG tag source specifically for that post.
The post option works through this logic until it gets a result