Generate preview images with custom text?

I´m working on a webshop where you can personalize Glasses, Pens etc. with gravures...

We also want a "Preview" function like in this shop (click on "Vorschau"): http://ift.tt/1H41JWW

They are generating the preview images through this link: http://ift.tt/1gnSFHj

What is a .dtm file and how can I implement this feature on my website?

Thanks in advance!