Unfortunately it's measuring at 273px, so it's above the limit! Taking a peek at your signature's BB code, the main problem is you're using both the width and height options for the image. XenForo's a bit weird where when both options are used for an image, it'll only use the width option by default and auto-scale the height, which is what's causing your problems. I'd just take the width option out and make the height 250px or less to solve it.