I will try to answer this question.
Unity has a XYZ coordinate system that can be translated to real world. I would measure real distances to these points (from the center where you took your picture) in your location/room and send these coordinates via backend to Unity3D client.
In Unity you can create Vector3
positions or directions based on coordinates you sent before. Use these positions/directions to instantiate 'hotspots' objects prefabs in right positions and directions. It might be necessary to adjust the scale/units to get the right result.
Once you have your 'hotspot' objects in place add a script to them that will load new scene (on click) with another location/image and repeat the process.
This is a very brief suggestion on how to do it. The code would be quite simple.