Sun and Sky ...
 

When I render an Image Plane as a background behind my geometry I get a darker grayed down image when using the “Sun and Sky” indirect lighting source. Any other lighting source works fine. Why is this and how can I correct it?

your image plane might be casting shadows

check that by going into render stats in the attribute editor

you need to add a gamma correction node to your texture.

Hello mtcgs, not sure how helpful this will be but you could try assigning a surface shader to your background image plane..then map your texture to the Out Color attribute. That way, it won't be affected by lights or shadows.

try to render your scene in a different passes and composite the results in after effects or digital fusion or any other correspondant.