please help.....I don't even know what it is called that I am looking for or how to go about creating it......

I am a screen printer, tee shirt images that are customized with the customer's name, ect.

I have graphic images in the background. These graphics have text. I am looking for the means to allow the user to edit their text fields and display it on screen. There are dozen of websites that have this capability, but I don't know what it is or called or how to start researching.