Shaders and postprocessing won’t be necessary in this case. 
If you want the image to maintain a position in 3D space (for example the flare above Earth), it should be enough to create a Sprite element and add it as a child to the Earth object.
It is possible to calculate screen width & height and then position the image relatively to any corner of the screen - but I’d expect it to get very messy, very quickly, when you’ll attempt to align 2D images to 3D meshes on the scene.