Compositing result does not show up in uv image editor

hello,

i am trying to mask an image.
therefore i have already made an mask image. (masked out windows in an interior scene)
Now i want to color balance this image again. (afterwards i want to paste in an alpes background)
However, the image doesnt show up anymore in the uv image editor window.
Only in the node editor window when i activate backdrop.
What is a render layer? And what an image input? To which end node do i connect them correctly?
i mean composite node and viewer node.

Any clarification is welcome.

Thanks

Add a viewer node to the output you want to see and set the image editor to the viewer node

What is a render layer? And what an image input?
Render layer is a what you get when you render the scene (F12). You can set different render layers to render different layers in your scene and output various passes.
Image Output node = output the result of the input to a saved image file
Composite node = the result of rendering and passing the render through the node setup. Basically the result of pressing F12. This does not save the result to a saved file.

Read the manual for info about all the composite nodes

Aha.
Can i load any image into the render layer?