After a lot of research, I finally gave up and decided to ask you guys how to do it... I remember seeing something about using Mental Ray's glare shader to add the effect to pre-rendered images. The technique consisted, as far as I can remember, of putting an existing image as BG and adding the glare shader to the renderer. Well, I did that and my result was... nothing. I was able to have objects (models, not those in the BG pic) glowing the way they should, but the effect was not affecting the background, which is my goal. Not many parameters I could change (Glare does not have much to mess with), so I really have no idea what I'm missing here.
Any thoughts?