iOS app for decorating Virtual Reality rooms I'm looking for a simple (or complex) iOS app that will allow me to put some pictures in virtual "rooms" (like an art gallery) that can then be viewed on the web or smartphones. That's all.

I think one of the many interior home decoration apps might do the trick but I don't know which is the simplest for my very narrow use case. I don't care about creating or placing any 3D models. I only want to hang some virtual paintings on some virtual walls and let other people virtually walk through them.
Your best bet is Aurasma. It is a simple yet powerful augmented reality platform, and will do what you need and more. Should you want to add videos and 3D 'realities' to the virtual space, you can do it with ease. It is simple enough for Middle School children to use effectively and powerful enough for professionals.