To achieve that effect you desire you will need to create a Hybrid Character. This is a combination of a morph (photo) based and sprite (vector) based head.

An example of this is included with CTA. You can see how it works by loading Mr Rosenberg and switching to Composer Mode. From here go to Head - Head and then select Human - Hybrid - Mr Rosenberg Hybrid.

You will then see that this character is made up of a human head from a photo with an additional spite used for the mouth. To create your own hybrid head you will need to do a little work in an image editor first to prepare your character.

This will involve cutting out the mouth of your photo and saving it as a separate image file. Then you can create your new character from the original photo and then add the extra mouth in Composer Mode.

If you haven't already I would take a look at the tutorial below which decribes the different kinds of characters you can create in CrazyTalk Animator.