Images cannot be dynamically loaded, so I'm not sure what you are even asking. in such a scenario JSON could only control the visibility of pre-existing layers, but the layers themselves would already have to exist inside the composition. I think you have a wrong understanding how this works and as far as your vague explanation allows to understand things, replacing the footage conventionally or using a script would be much more efficient for what you seem to intend on doing. It's not even clear how and where you even want to dial in your selector value, so you would need to be much more specific. In any case, while it may be possible to do with expressions, I can't see the use case here. The expressions themselves would likely be quite complex, you'd have bloated comps with potentially tens or hundreds of unused layrs also controlled by expressions and for that reason alone the project may become terribly slow. it may be advisable to realyl explore other options.