I have a saved image from yesterday that I wanted to do some post processing for, but when I drop the image into the image editor the image doesn’t appear in the node editor (I have clicked on enable nodes and compositing). How do I go back and post process old saved images like this?
Also, how do I get the image into the background of the node editor 
The beginning of this tutorial shows you how to set up a video-clip (or image) in the compositing nodes.
Perfect, thanks mate.
I have a related question though if I may, with the render layers node I can use “mist” is it possible for the image node?
Not entirely sure what you’re asking me.
A Jpeg or a png or whatever does not have a built in “mist” layer.
Gleb here at 2:32 adds an interesting effect… the whole video is interesting in fact, but this is the part I’m talking about
And using the color ramp to adjust the fall off
Is the image you’re compositing created in Blender? If so you can render at 1 sample to generate a mist node, save that as an image, and bring it into your compositing blend.
There are cameras around that generate depth info, but it is not a standard feature of most image formats.
It’s created in blender yes, but I’ve moved the camera, changed the perspective and depth of field… I’m not convinced I can get it back the same to render again at 1 sample.
Well, you can always re-render, and that can be your input node.
BTW - once you’ve done some post pro in compositing nodes and you want to save the image, you don’t have to render again. Open up an image editor panel as well as the node panel, change one node (and probably change back again) and the image will update and you can save that.