I don’t have any experience programming shaders or doing any form of postprocessing but I would like to know how I would go about overlaying an image onto a scene while maintaining its aspect ratio even after the window is resized, the image size should remain static, similar to how a 3d object appears in a scene. For example, in this image
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.
Thank you, I’ll I’ll definitely give it a try, I never worked with sprites before either, can you apply blend filters onto them?
Sure, under the hood Sprite is basically a plane geometry with a texture applied onto it. You can modify its material the same way you’d modify any other.
