Help with the Layered Image Editor

I recently downloaded the following freebie by Mephoria:
https://www.daz3d.com/forums/discussion/216141/new-dforce-garments-casual-gym-for-g2f
It is a great item but I need some help putting my own pictures on the shirt.
I load the shirt and open its LIE but I don't know how to use it to transfer an image from my desktop onto the shirt.
Can anyone help me do that?
Comments
DS is rendering, so from memory: