custom last name feature for T shirt store

Which kind of technology does the following sites use to afford a feature for the user to insert his own custom "LAST NAME" and get the same reflect on the Tshirt mockup without disturbing the image? As you see, the given TEXT gets merged to the t-shirt perfectly. Im trying to setup my own opencart tshirt store with this above mentioned feature.

http://ift.tt/1nGZCGv http://ift.tt/1S0I84O