When oneboxing SE blog post, the image overflows:
The image is too tall, and there's a strange white border around it… (taken from the blog image)
When hovering over it, it becomes even more clear due to the dotted border.
The image on the blog should have margins, rather than white pixels, and the onebox image should have a max-height. The overflow in chat is only apparent when you hover over the post, showing the dotted border.
I'm on Safari 9, OS X 10.11.