You've already read this post, but let me add another book to the list of books that feature fat girls and don't picture them on the cover:
If you've read the book, you know that the point Daelyn's at in the book, she's not fat. Page 111 she makes it clear, though, she's not skinny. She's been bullied for her weight forever, and she says (likely warped in her sense of perception) she's not attractive at all.
I REALLY don't think it's asking for a whole lot to be a little more realistic about this.