It's probably inheriting a background colour from the css. Which theme are you using?
ETA: Ok, no. It's your image host remaking the image with a black background. Told you to use another host.
OK. I fixed it, by simply using the correct image out of the two. I'm sure that's what I set it to in the first place.
Never mind now.
On a side note, Os: I see you edited your post after I edited mine. When you were submitting your post, it clearly did not warn you that I had -- in the meantime -- edited my post. If you really wanna be an awsome dude, you could write some code to make it warn a responder that the last post was edited in the meantime.
It would be useful in many debates here too.