Quote - did you make a true HDR out of the image? to have genuine IBL ?

If I understand the whole thing correctly: Yes.
I used the image (sunset) I had, doubled the canvas width, mirrored the sunset, then increased the height, filling the bottom 48% of the image with black. Overall size of the image used was about 6000X4000 pixels.
When it asked if I wanted to have lighting calculated automatically, I clicked yes.
Since this image, I've rendered a skydome without any objects present, then re-inserted it into the atmosphere environment and am rendering it again 

Quote - did you make a true HDR out of the image? to have genuine IBL ?

...Insert some witty or thought provoking comment here...