I would like to get notified as soon as the engine renders an image (after all the post processing is done).
Also, I would like to inspect each RGBA pixel in that image.
How can I do this?
See if this can help: Render to Texture Toolset Setup | Unreal Engine Documentation
I am not sure what you are trying to do but based on your previous questions I would gently suggest you look for a dedicated rendering software like 3DStudio, Maya or Blender.
UE4 provides pixel inspector which provides even more information then RGBA raw screenshot would give you, to access it go to Windows->Devlopment Tools->Pixel Inspector and then start inspecting.
If you want you want you can try to look up in to Unreal Enigne source code and see Pixel Inspector gets this data, by simple look on Widget Reflector you can find out that code for is in class SPixelInspector and entire thing as it’s own code module with same name. So go to github and search.