It was April 20's one, you can view it now on the daily email's page and change the date. The user has removed the photo from her blog though, due to user complaints.
Given it was a joke about flashing, the image of some people, nude, from the back, was entirely in context.
But Spark does have a community guideline that states no nudity. The user has complied. End of issue, really.
Perhaps spark should be a little more careful in picking member's blogs to promote and ensuring the images contained within do not violate the CG. But then, perhaps no actual human being is involved in this process and simply a highly-voted blog is randomly chosen each day. That makes a lot more sense, really, and would explain it.
Personaly I found it tasteful and not offensive.
Deb, in New Zealand