I think that sometimes thinner people might not understand the depth of the sensitivity that others have about their weight. She might just think it is a really cute picture (and does not understand you genuinely are uncomfortable with pics).
I can think of a lot of scenarios when people might say not to post a pic because of a bad hair day or something...but if someone posts it, you can all have a laugh at it (i.e., sometimes the request not to post a pic is not really dead serious). Weight is different because it is such a sensitive topic. For someone who does not struggle with their weight, she may see no good reason not to post it...she may think you look perfectly fine! We think about our own appearances much more than others think about our appearance.
It was not very nice of her to post the pic when you asked her not to, though; I agree with others that you should ask her if she will take it down because you really don't like pictures of you posted, and to please not do it in the future. If she refuses to take it down (and she has done petty things in the past), that may be a sign to me that it is malicious.


