How to use 2D image instead of ThingMark in ThingWorx studio to build AR experience

I am able to use PTC provided ThingMark to build AR experience, however, similar to Vuforia, how do I use my custom image as initialization target to build an AR experience? 

Right now, I can only see three options: Model Targets, ThingMark and Spatial Targets as initialization targets for building an AR experience.