I imported images as panes to create mist effect using PNG images. These are not appearing in the final render but are appearing in the viewport in render mode.
When you drag an image into the viewport, or use Add > Image > Reference, or Add > Image > Background, those images will have no real geometry (they are just an empty with an image associated to it), and as such they are not renderable. They are just reference.
Use Add > Image > Images as Planes (or File > Import > Images as planes) to have an object that has 4 vertices and a face with the image used as a texture on the material.