As stated on the page where you upload your image:

"The rules are simple: only upload JPEG, PNG, or GIF. No larger than 80 kB. Suggested maximum width is 500 pixels. We will take away this privilege if what you post is copyrighted or obscene. "

Maybe you violated the width limit?

Update: Hmmm. If your current node image is the same one, it doesn't seem to violate -- 35 kb and 437 pixels wide ... I'm stumped. Maybe one of the janitors or gods could tell you ... or possibly it was (my favorite theory for pretty much anything) evil gnomes.