Describing an artists impression of something that's not visible as "a diagram" doesn't sit well with me. I fear many people will see this and assume it's actual photography - my first thought was "how the hell did they manage to get that image" followed swiftly by "oh, it's a mockup; move along".
"A supermassive black hole is depicted in this artist's concept, surrounded by a swirling disk of material falling onto it. The purplish ball of light above the black hole, a feature called the corona, contains highly energetic particles that generate X-ray light. If you could view the corona with your eyes, it would appear nearly invisible since we can't see its X-ray light. "
I agree, when I read "diagram" I don't think "photograph." This diagram is more elaborate some others, but I don't think the first impression is to assume it is a real photo when it is explicitly called otherwise.
From the full image link, http://www.jpl.nasa.gov/spaceimages/details.php?id=pia20051 :
"A supermassive black hole is depicted in this artist's concept, surrounded by a swirling disk of material falling onto it. The purplish ball of light above the black hole, a feature called the corona, contains highly energetic particles that generate X-ray light. If you could view the corona with your eyes, it would appear nearly invisible since we can't see its X-ray light. "