So to get white letters from an image onto a glass surface you're going to add this to the opacity section of your material. This will give the letters the same materialistic feel as the glass (i.e colour and reflectivity).
However, should you wish to keep these two parts visually sperate, make a duplicate of the glass mesh you have and place it slightly above the position of the glass. Then, make a new physical material and add your image to the new materials opacity rather than the glass materials. This will allow you to freely edit the look of the glass without effecting the look of the letters.
I've created a scene with both examples being used: https://clara.io/editor/a65b424e-c982-4f35-b141-04573b4e93b5
Hope this helps,
Riley from Exocortex