Hi @dzcannon ,
There are several ways to go about this, also depending on what you want exactly…
For instance, you can use this Node setup for the BG Image:

You can of course also use an Emission Shader, in stead of a Diffuse for the BG Image…
Looking through the Camera, with these World Settings (I removed the window’s shadow here):

I turned off the World Glossy, so it won’t reflect in the mirror, but that will probably not be necessary for you, if your BG Image covers the whole window…
Explaining ‘everything’ about the Light Path Node might be a cool idea for the next Discord Q&A Livestream in November…