It is heat transfers. U can easily buy them from shops or online. I don't know about the process how they are produced but I do know how to apply them to your own shirts. The company I work for sells similar patterns here, just ours are rhinestones, not this kind of images.
If you want to order these transfers online, try to search "heat transfer" and you'll have many results.
Now I'm going to tell you how to process with your shirts:
1. Turn on your iron to the hottest setting and wait until it gets heated. Remember to disable the steam setting and use a dry iron.
2. Press your iron onto your T-shirt to get it heated, just the area you want your transfer to be placed. Then put the transfer on that area.
3. Press the iron on the transfer lightly and stay still for 3 to 5 seconds.
4. Change the iron to a cooler setting. Slowly move the iron around over the transfer until it firmly sticks onto the shirt. You can check if the edges are closely secured to the shirt while doing this.
Hope above will help you.